[NFC, Refactor] Convert TypeSpecifiersPipe from Specifiers.h to a scoped enum (tiny)

[tschuett]

Summary

ninja clang clangd (with assertions)

[aaron.ballman]

LGTM! Would you like me to commit this one on your behalf or would you like to request your own commit access (https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access)?

[tschuett]

I am happy, if you could commit this for me. I am still learning.

[aaron.ballman]

In D91506#2397158, @tschuett wrote:

I am happy, if you could commit this for me. I am still learning.

I'm happy to do so -- how would you like me to attribute the patch? Is Thorsten <[e-mail]> what you'd like me to use?

[tschuett]

Yes. Thank you!

[aaron.ballman]

In D91506#2397181, @tschuett wrote:

Yes. Thank you!

You're welcome! I've commit on your behalf in a6ac2b32fbab9679c8f2fa97a3b1123e3a9654c8, thank you for the patch!

clang/include/clang/Basic/Specifiers.h

namespace clang {
 
   /// Specifies the signedness of a type, e.g., signed or unsigned.
   enum TypeSpecifierSign {
     TSS_unspecified,
     TSS_signed,
     TSS_unsigned
   };
 
-  enum TypeSpecifiersPipe {
-    TSP_unspecified,
-    TSP_pipe
-  };
+  enum class TypeSpecifiersPipe { Unspecified, Pipe };
 
   /// Specifies the kind of type.
   enum TypeSpecifierType {
     TST_unspecified,
     TST_void,
     TST_char,
     TST_wchar,        // C++ wchar_t
     TST_char8,        // C++20 char8_t (proposed)

clang/lib/Sema/DeclSpec.cpp

if (TypeSpecType == TST_error)
     return false;
   if (TypeSpecType != TST_unspecified) {
     PrevSpec = DeclSpec::getSpecifierName((TST)TypeSpecType, Policy);
     DiagID = diag::err_invalid_decl_spec_combination;
     return true;
   }
 
   if (isPipe) {
-    TypeSpecPipe = TSP_pipe;
+    TypeSpecPipe = static_cast<unsigned>(TypeSpecifiersPipe::Pipe);
   }
   return false;
 }
 
 bool DeclSpec::SetTypeAltiVecPixel(bool isAltiVecPixel, SourceLocation Loc,
                           const char *&PrevSpec, unsigned &DiagID,
                           const PrintingPolicy &Policy) {
   if (TypeSpecType == TST_error)