[clang] Deprecate -fheinous-gnu-extensions; introduce a new warning flag (PR #105821)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-x86_64-linux-abi-test` running on `sie-linux-worker2` while building `clang` at step 7 "abi-test-suite".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/8/builds/3716

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (abi-test-suite) failure: ABI test-suite completed (failure)
******************** TEST 'SN C++ IA64 ABI Tests :: struct_layout_tests/PACKED/T_Snnn_xbg.c' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
TEST PASSED. 3000 classes. 22595 tests. 0 failures.

--
Command Output (stderr):
--
RUN: at line 4: clang -c -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1.o -I "/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common" "/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common/testsuite.c"
+ clang -c -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1.o -I /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common/testsuite.c
/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common/testsuite.c:205:24: warning: comparison between pointer and integer ('VTBL_ENTRY' (aka 'long') and 'void *') [-Wpointer-integer-compare]
  205 |             if (act[j] == NULL) {
      |                 ~~~~~~ ^  ~~~~
/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common/testsuite.c:315:11: warning: cast to smaller integer type 'int' from 'VTBL_ENTRY *' (aka 'long *') [-Wpointer-to-int-cast]
  315 |           (int)(*(VTBL_ENTRY**)(b->ofst+(char*)var_ptr))  ,
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common/testsuite.c:316:15: warning: cast to smaller integer type 'int' from 'VTBL_ENTRY *' (aka 'long *') [-Wpointer-to-int-cast]
  316 |               (int)cd->vtbl.var ,(int) b->num_negative_vtbl_entries, (int)b->vtbl_ofst,
      |               ^~~~~~~~~~~~~~~~~
/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common/testsuite.c:317:15: warning: cast to smaller integer type 'int' from 'VTBL_ENTRY *' (aka 'long *') [-Wpointer-to-int-cast]
  317 |               (int)(cd->vtbl.var + (b->vtbl_ofst+b->num_negative_vtbl_entries)));
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 warnings generated.
RUN: at line 5: clang -c /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/T_Snnn_xbg.c -I "/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common" -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp2.o
+ clang -c /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/T_Snnn_xbg.c -I /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp2.o
RUN: at line 6: clang++ -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1%exeext  /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1.o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp2.o
+ clang++ -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1%exeext /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1.o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp2.o
RUN: at line 7: /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1%exeext | grep "TEST PASSED"
+ /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp1%exeext
+ grep 'TEST PASSED'
RUN: at line 8: clang -x c++ -frtti -c /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/T_Snnn_xbg.c -I "/home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common" -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp2.o
+ clang -x c++ -frtti -c /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/T_Snnn_xbg.c -I /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp2.o
fatal error: error in backend: IO failure on output stream: No space left on device
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: clang -x c++ -frtti -c /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/T_Snnn_xbg.c -I /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/common -o /home/buildbot/buildbot-root/abi-test/test-suite/ABI-Testsuite/test/struct_layout_tests/PACKED/Output/T_Snnn_xbg.c.tmp2.o
1.	<eof> parser at end of file

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/105821