[Parser] Fix attr infloop on "int x [[c"

[sammccall]

Summary

Similar to ad2d6bbb1435cef0a048c9aed3dcf9617640f222

[aaron.ballman]

LGTM, thank you for the fix!

clang/lib/Parse/ParseDeclCXX.cpp

if (Tok.is(tok::kw_using)) {
     }
     if (!TryConsumeToken(tok::colon) && CommonScopeName)
       Diag(Tok.getLocation(), diag::err_expected) << tok::colon;
   }
 
   llvm::SmallDenseMap<IdentifierInfo*, SourceLocation, 4> SeenAttrs;
 
   bool AttrParsed = false;
-  while (!Tok.isOneOf(tok::r_square, tok::semi)) {
+  while (!Tok.isOneOf(tok::r_square, tok::semi, tok::eof)) {
     if (AttrParsed) {
       // If we parsed an attribute, a comma is required before parsing any
       // additional attributes.
       if (ExpectAndConsume(tok::comma)) {
         SkipUntil(tok::r_square, StopAtSemi | StopBeforeMatch);
         continue;
       }
       AttrParsed = false;

clang/test/Parser/cxx-attributes.cpp

 
 [[deprecated([""])]] int WrongArgs; // expected-error {{expected variable name or 'this' in lambda capture list}}
 [[,,,,,]] int Commas1; // ok
 [[,, maybe_unused]] int Commas2; // ok
 [[maybe_unused,,,]] int Commas3; // ok
 [[,,maybe_unused,]] int Commas4; // ok
 [[foo bar]] int NoComma; // expected-error {{expected ','}} \
                          // expected-warning {{unknown attribute 'foo' ignored}}
+// expected-error@+2 2 {{expected ']'}}
+// expected-error@+1 {{expected external declaration}}
+[[foo