Change Preprocessor::SkippingContents into Lexer::LexingRawMode. Raw mode

is an intra-lexer property, not a inter-lexer property, so it makes sense
for it to be define here.  It also makes no sense for macros, and allows us
to define it more carefully in the header.

While I'm at it, improve comments and structuring in Lexer.h

llvm-svn: 38701