Hide pragma handler classes in ParsePragma.cpp

This reduces the number of files we need to touch to add a new pragma,
and reduces the number of externally visible symbols in clang.

Make the handlers structs instead of classes because the vast majority
have no private members.

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D2834

llvm-svn: 201820