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