Improve diagnostics if _Noreturn is placed after a function declarator. (This sometimes happens when a macro is used that expands to either the GNU noreturn attribute or _Noreturn.)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221630 91177308-0d34-0410-b5e6-96231b3b80d8