Preprocessor: Recover instead of mutating a token in ExpandBuiltinMacro

We would CreateString on arbitrary garbage instead of just skipping to
the end of the builtin macro.  Eventually, this would cause us to crash
because we would end up replacing the contents of a character token with
a numeric literal.

This fixes PR21825.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224238 91177308-0d34-0410-b5e6-96231b3b80d8