[clang,flang] Add help text for -fsyntax-only

Fix for the problem with displaying options `-fsyntax-only` in clang and flang-new in help
Fix https://github.com/llvm/llvm-project/issues/57033

Before:
``` $ clang  -help | grep syntax
  -objcmt-migrate-property-dot-syntax
         Enable migration of setter/getter messages to property-dot syntax
```
After:
```
 $ clang -help | grep syntax
  -fsyntax-only           Run the preprocessor, parser and semantic analysis stages
  -objcmt-migrate-property-dot-syntax
         Enable migration of setter/getter messages to property-dot syntax
```

Reviewed By: vzakhari, awarzynski, MaskRay, alexiprof

Differential Revision: https://reviews.llvm.org/D131808