Let the CompilerInstance create our clang ASTContext

Summary:
Now that we moved the BuiltinContext and SelectorTable to the
CompilerInstance, we can also get rid of manually creating our
own ASTContext, but just use the one from the CompilerInstance
(which will be created with the same settings).

Reviewers: vsk, aprantl, davide

Reviewed By: davide

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D51253

llvm-svn: 340748