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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59723 91177308-0d34-0410-b5e6-96231b3b80d8