Change how raw lexers are handled: instead of creating them and then

using LexRawToken, create one and use LexFromRawLexer.  This avoids
twiddling the RawLexer flag around and simplifies some code (even 
speeding raw lexing up a tiny bit).

This change also improves the token paster to use a Lexer on the stack
instead of new/deleting it. 

llvm-svn: 57393