Change -verify mode to find the "expected-error" and "expected-warning" strings

Change -verify mode to find the "expected-error" and "expected-warning" strings
with a raw lexer instead of a PP lexer.  This means that -verify doesn't scan
#include'd headers for expected-error/warning strings, and it also means that it
doesn't ignore them in #if 0.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59774 91177308-0d34-0410-b5e6-96231b3b80d8