[clang] [clang][OpenMP] Only accept char strings in the message clause (PR #226080)

[Douglas Yung via cfe-commits]

dyung wrote:

@akash-manna-sky, the test you added is failing on the PS4 bot, it is likely due to the PS4 target having a default C standard that is older than other targets. Can you take a look?

https://lab.llvm.org/buildbot/#/builders/144/builds/66294
```
******************** TEST 'Clang :: OpenMP/error_message.c' FAILED ********************
Exit Code: 1
Command Output (stdout):
--
# 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/include -nostdsysteminc -verify -fopenmp -fopenmp-version=51 -ferror-limit 100 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/error_message.c
# executed command: /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/include -nostdsysteminc -verify -fopenmp -fopenmp-version=51 -ferror-limit 100 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/error_message.c
# .---command stderr------------
# | error: 'expected-error' diagnostics expected but not seen: 
# |   File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/error_message.c Line 24: foo
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/error_message.c Line 24: use of undeclared identifier 'u8'
# |   File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/error_message.c Line 24: expected ')'
# |   File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/error_message.c Line 24: ERROR
# | error: 'expected-note' diagnostics seen but not expected: 
# |   File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/error_message.c Line 24: to match this '('
# | 5 errors generated.
# `-----------------------------
# error: command failed with exit status: 1
--
********************
```

https://github.com/llvm/llvm-project/pull/226080