[clang][parse] Remove dead ProhibitAttributes() call

[tbaeder]

Summary

Since MaybeParseGNUAttributes() only takes a ParsedAttributes and not a ParsedAttributesWithRange, the source range of attrs will always be invalid after calling MaybeParseGNUAttributes(). That means the call to ProhibitAttributes() removed in this patch will always simply do nothing.

The comment that's removed alongside the ProhibitAttributes() call seems to be incorrect as well since numerous tests use (and expect) GNU-style attributes in enum bodies.

[aaron.ballman]

I did some digging and GCC used to reject attributes in this position but started accepting them in GCC 6, so this is the correct change. Thanks for catching it! LGTM

clang/lib/Parse/ParseDecl.cpp

if (Tok.isNot(tok::identifier)) {
       break;
     }
     IdentifierInfo *Ident = Tok.getIdentifierInfo();
     SourceLocation IdentLoc = ConsumeToken();
 
     // If attributes exist after the enumerator, parse them.
     ParsedAttributesWithRange attrs(AttrFactory);
     MaybeParseGNUAttributes(attrs);
-    ProhibitAttributes(attrs); // GNU-style attributes are prohibited.
     if (standardAttributesAllowed() && isCXX11AttributeSpecifier()) {
       if (getLangOpts().CPlusPlus)
         Diag(Tok.getLocation(), getLangOpts().CPlusPlus17
                                     ? diag::warn_cxx14_compat_ns_enum_attribute
                                     : diag::ext_ns_enum_attribute)
             << 1 /*enumerator*/;
       ParseCXX11Attributes(attrs);
     }