Standardize the parsing of function type attributes in a way that

Standardize the parsing of function type attributes in a way that
follows (as conservatively as possible) gcc's current behavior:  attributes
written on return types that don't apply there are applied to the function
instead, etc.  Only parse CC attributes as type attributes, not as decl attributes;
don't accepet noreturn as a decl attribute on ValueDecls, either (it still
needs to apply to other decls, like blocks).  Consistently consume CC/noreturn
information throughout codegen;  enforce this by removing their default values
in CodeGenTypes::getFunctionInfo().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@95436 91177308-0d34-0410-b5e6-96231b3b80d8