Change the Lexer ctor used to lex _Pragma directives into a static factory

method.  This lets us clean up the interface and make it more obvious that
this method is *really really* _Pragma specific.

Note that _Pragma handling uglifies the Lexer in the critical path.  It would
be very interesting to consider making _Pragma remapping be a new special
lexer class of its own.

llvm-svn: 62425