Parse: Diagnose malformed 'message' arguments for 'availability' attr

The parsing code for 'availability' wasn't prepared for string literals
like "a" L"b" showing up.  Error if this occurs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213350 91177308-0d34-0410-b5e6-96231b3b80d8