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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62425 91177308-0d34-0410-b5e6-96231b3b80d8