Add ugly "test harness" for PTHLexer that is not enabled by default. The

(temporary hack) to test the PTHLexer is that whenever we would create a Lexer
object we instead raw lex a memory buffer first and then use the PTHLexer. This
logic exists only to driver the PTHLexer and will be removed/changed in the
future. Note that the regular path using normal Lexer objects is what is used by
default.

llvm-svn: 59723