[Preprocessor] Prevent expansion of y in x ## y when x is empty

When x is empty, x ## is suppressed, and when y gets expanded, the fact that it follows ## is not
available in the macro expansion result. The macro definition can be checked instead, the ## will
be available there regardless of what x expands to.

Fixes http://llvm.org/PR12767

Patch by Harald van Dijk!

llvm-svn: 182699