Fix r173881 to properly skip invalid UTF-8 characters in raw lexing and -E.

This caused hangs as we processed the same invalid byte over and over.

<rdar://problem/13115651>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173959 91177308-0d34-0410-b5e6-96231b3b80d8