[clang] Improved isSimpleTypeSpecifier (#79037)

- Sema::isSimpleTypeSpecifier return true for _Bool in c99 (currently
returns false for _Bool, regardless of C dialect). (Fixes #72203)
- replace the logic with a check for simple types and a proper check for
a valid keyword in the appropriate dialect

Co-authored-by: Carl Peto <CPeto@becrypt.com>