Parse: Concatenated string literals should be verified in inline asm

While we would correctly handle asm("foo") and reject asm(L"bar"), we
weren't careful to handle cases where an ascii literal could be
concatenated with a wide literal.

This fixes PR21822.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223992 91177308-0d34-0410-b5e6-96231b3b80d8