[clang] Adjust -pedantic-errors -WX/-Wno-error=X interaction (#184756)
While -Wno-long-long suppresses -pedantic-errors diagnostics in both GCC
and Clang, GCC -Wno-error=long-long emits warnings while Clang still
emits errors.
```
% echo 'long long x = 0;' | gcc -std=c89 -pedantic-errors -Wno-error=long-long -x c -fsyntax-only -
<stdin>:1:6: warning: ISO C90 does not support 'long long' [-Wlong-long]
% echo 'long long x = 0;' | clang -std=c89 -pedantic-errors -Wno-error=long-long -x c -fsyntax-only -
<stdin>:1:1: error: 'long long' is an extension when C99 mode is not enabled [-Werror,-Wlong-long]
1 | long long x = 0;
| ^
1 error generated.
```
The order of -pedantic-errors and -Wno-error=long-long does not matter.
Two fixes to how extension diagnostics interact with -pedantic-errors
in getDiagnosticSeverity():
1. -Wno-error=X now downgrades -pedantic-errors diagnostics to warnings.
setDiagnosticGroupWarningAsError() sets HasNoWarningAsError but not
IsUser, so the ExtBehavior upgrade would re-promote to error. When
HasNoWarningAsError is set, cap the ExtBehavior upgrade at Warning.
2. -Wfoo no longer prevents -pedantic-errors from upgrading to error.
Previously, -Wfoo set IsUser which blocked the ExtBehavior upgrade
entirely. Now only -Wno-foo (IsUser + Ignored) blocks the upgrade,
matching GCC behavior.
```
% echo 'long long x = 0;' | myclang -std=c89 -pedantic-errors -Wno-error=long-long -x c -fsyntax-only -
<stdin>:1:1: warning: 'long long' is an extension when C99 mode is not enabled [-Wlong-long]
1 | long long x = 0;
| ^
1 warning generated.
```
Fixes https://github.com/llvm/llvm-project/issues/184250
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