[clang] fix bug that undefined internal is a warning only for -pedantic-errors (PR #98016)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-ubuntu-fast` running on `sie-linux-worker` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/2080

Here is the relevant piece of the build log for the reference:
```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: Sema/undefined-internal-basic.c' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/19/include -nostdsysteminc -fsyntax-only -verify /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c -Wno-pointer-arith -Wno-gnu-alignof-expression -Wno-unused -pedantic-errors
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/19/include -nostdsysteminc -fsyntax-only -verify /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c -Wno-pointer-arith -Wno-gnu-alignof-expression -Wno-unused -pedantic-errors
error: 'expected-error' diagnostics expected but not seen: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 3: function 'a' has internal linkage but is not defined
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 4: function 'b' has internal linkage but is not defined
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 5: function 'c' has internal linkage but is not defined
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 6: function 'd' has internal linkage but is not defined
error: 'expected-error' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 13: '_Alignof' is a C11 extension
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 15: '_Generic' is a C11 extension
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 17: '_Generic' is a C11 extension
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 22: '_Generic' is a C11 extension
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 24: '_Generic' is a C11 extension
error: 'expected-note' diagnostics expected but not seen: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 11: used here
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 17: used here
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 22: used here
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/Sema/undefined-internal-basic.c Line 24: used here
13 errors generated.

--

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


```

https://github.com/llvm/llvm-project/pull/98016