Unify __declspec attribute argument parsing with the common attribute argument parsing code.

This removes a diagnostic that is no longer required (the semantic engine now properly handles attribute syntax so __declspec and __attribute__ spellings no longer get mismatched). This caused several testcases to need updating for a slightly different wording.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205234 91177308-0d34-0410-b5e6-96231b3b80d8