[flang] [llvm] [flang] implement show_descriptor intrinsic, a non-standard extension (PR #169137)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-x86_64-windows` running on `minipc-ryzen-win` while building `flang-rt,flang` at step 8 "test-build-unified-tree-check-flang-rt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/166/builds/4535

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

```
Step 8 (test-build-unified-tree-check-flang-rt) failure: test (failure)
...
   79 |   if (const ::testing::AssertionResult gtest_ar = (expression)) \
      |                                                    ^
1 warning generated.
618.845 [32/6/295] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Complex.cpp.obj
620.282 [31/6/296] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/CharacterTest.cpp.obj
620.510 [30/6/297] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.dynamic_dbg.dir/matmul-transpose.cpp.obj
621.137 [29/6/298] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/CrashHandlerFixture.cpp.obj
622.397 [28/6/299] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.dynamic_dbg.dir/findloc.cpp.obj
624.370 [27/6/300] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Derived.cpp.obj
625.845 [26/6/301] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Descriptor.cpp.obj
FAILED: [code=1] flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Descriptor.cpp.obj 
C:\buildbot\flang-x86_64-windows\build\bin\clang-cl.exe --target=x86_64-pc-windows-msvc  /nologo -TP -DGTEST_HAS_RTTI=0 -DGTEST_NO_LLVM_SUPPORT=1 -DLLVM_BUILD_STATIC -DNOT_EXE=\"C:/buildbot/flang-x86_64-windows/build/bin/not.exe\" -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_NO_ASSERTIONS -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -IC:/buildbot/flang-x86_64-windows/llvm-project/third-party/unittest/googletest/include -IC:/buildbot/flang-x86_64-windows/llvm-project/third-party/unittest/googlemock/include -IC:/buildbot/flang-x86_64-windows/llvm-project/flang-rt/include -IC:/buildbot/flang-x86_64-windows/llvm-project/flang-rt/../flang/include /DWIN32 /D_WINDOWS   /Zc:inline /Zc:__cplusplus /Oi /Brepro /bigobj /permissive- -Werror=unguarded-availability-new /W4 -wd4141 -wd4146 -wd4244 -wd4267 -wd4291 -wd4351 -wd4456 -wd4457 -wd4458 -wd4459 -wd4503 -wd4624 -wd4722 -wd4100 -wd4127 -wd4512 -wd4505 -wd4610 -wd4510 -wd4702 -wd4245 -wd4706 -wd4310 -wd4701 -wd4703 -wd4389 -wd4611 -wd4805 -wd4204 -wd4577 -wd4091 -wd4592 -wd4319 -wd4709 -wd5105 -wd4324 -wd4251 -wd4275 -w14062 -we4238 /Gw /O2 /Ob2  -std:c++17 -MD  /EHs-c- /GR- -UNDEBUG -Xclang --dependent-lib=C:/buildbot/flang-x86_64-windows/build/lib/clang/22/lib/windows/clang_rt.builtins-x86_64.lib /showIncludes /Foflang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Descriptor.cpp.obj /Fdflang-rt\unittests\Runtime\CMakeFiles\RuntimeTests.dir\ -c -- C:/buildbot/flang-x86_64-windows/llvm-project/flang-rt/unittests/Runtime/Descriptor.cpp
C:/buildbot/flang-x86_64-windows/llvm-project/flang-rt/unittests/Runtime/Descriptor.cpp(188,5): error: use of undeclared identifier 'ftruncate'
  188 |     ftruncate(fileno(tmpf), 0);
      |     ^~~~~~~~~
1 error generated.
628.205 [26/5/302] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/InputExtensions.cpp.obj
629.361 [26/4/303] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/ExternalIOTest.cpp.obj
629.715 [26/3/304] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Format.cpp.obj
631.266 [26/2/305] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Inquiry.cpp.obj
653.335 [26/1/306] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.dynamic_dbg.dir/matmul.cpp.obj
ninja: build stopped: subcommand failed.
FAILED: [code=1] runtimes/CMakeFiles/check-flang-rt C:/buildbot/flang-x86_64-windows/build/runtimes/CMakeFiles/check-flang-rt 
C:\Windows\system32\cmd.exe /C "cd /D C:\buildbot\flang-x86_64-windows\build\runtimes\runtimes-bins && "C:\Program Files\CMake\bin\cmake.exe" --build C:/buildbot/flang-x86_64-windows/build/runtimes/runtimes-bins/ --target check-flang-rt --config Release"
ninja: build stopped: subcommand failed.
Cache directory:    C:\Users\buildbot-worker\AppData\Local\ccache
Config file:        C:\Users\buildbot-worker\AppData\Local\ccache\ccache.conf
System config file: C:\ProgramData\ccache\ccache.conf
Stats updated:      12/01/25 18:38:14
Local storage:
  Cache size (GB):    5.0 / 5.0 (100.1%)
  Files:            22327
  Hits:                 0
  Misses:               0
  Reads:                0
  Writes:               0

```

</details>

https://github.com/llvm/llvm-project/pull/169137