[AST] Don't include RecursiveASTVisitor.h in ASTContext.h

The untemplated implementation of getParents() doesn't need to be in a
header file.

RecursiveASTVisitor.h is full of repeated macro expansion.  Moving this
include to ASTContext.cpp speeds up compilation of
LambdaMangleContext.cpp, a small C++ file with few includes, from 3.7s
to 2.8s for me locally.  I haven't measured a full build, but it can't
hurt.

I had to fix a few static analyzer files that were depending on
transitive includes of C++ AST headers.

Reviewers: rsmith, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D982