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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I've reverted the changes in 9f283bfb9e66d57fefeb3a28318ec40081fa9a24 because of the bot breakage. Oddly, I am seeing a different failure on Windows locally:
```
FAIL: Clang :: Sema/undefined-internal-basic.c (1 of 1)
******************** TEST 'Clang :: Sema/undefined-internal-basic.c' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
f:\source\llvm-project\llvm\out\build\x64-debug\bin\clang.exe -cc1 -internal-isystem F:\source\llvm-project\llvm\out\build\x64-Debug\lib\clang\19\include -nostdsysteminc -fsyntax-only -verify F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c -Wno-pointer-arith -Wno-gnu-alignof-expression -Wno-unused -pedantic-errors
# executed command: 'f:\source\llvm-project\llvm\out\build\x64-debug\bin\clang.exe' -cc1 -internal-isystem 'F:\source\llvm-project\llvm\out\build\x64-Debug\lib\clang\19\include' -nostdsysteminc -fsyntax-only -verify 'F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c' -Wno-pointer-arith -Wno-gnu-alignof-expression -Wno-unused -pedantic-errors
# .---command stderr------------
# | error: 'expected-error' diagnostics expected but not seen:
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 3: function 'a' has internal linkage but is not defined
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 4: function 'b' has internal linkage but is not defined
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 5: function 'c' has internal linkage but is not defined
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 6: function 'd' has internal linkage but is not defined
# | error: 'expected-warning' diagnostics seen but not expected:
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 3: function 'a' has internal linkage but is not defined
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 4: function 'b' has internal linkage but is not defined
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 5: function 'c' has internal linkage but is not defined
# |   File F:\source\llvm-project\clang\test\Sema\undefined-internal-basic.c Line 6: function 'd' has internal linkage but is not defined
# | 8 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

********************
Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
********************
Failed Tests (1):
  Clang :: Sema/undefined-internal-basic.c
```
so something has gone amiss...


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