Recommit virtual file system

Previously reverted in r201755 due to causing an assertion failure.

I've removed the offending assertion, and taught the CompilerInstance to
create a default virtual file system inside createFileManager. In the
future, we should be able to reach into the CompilerInvocation to
customize this behaviour without breaking clients that don't care.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201818 91177308-0d34-0410-b5e6-96231b3b80d8
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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201820 91177308-0d34-0410-b5e6-96231b3b80d8