Lexer: Don't warn about Unicode in preprocessor directives.

This allows people to use Unicode in their #pragma mark and in macros
that exist only to be string-ized.

<rdar://problem/13107323&13121362>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174081 91177308-0d34-0410-b5e6-96231b3b80d8