[lldb] "See through" atomic types in ClangASTContext

Summary:
This enables us to display the contents of atomic structs. Calling the
removal of _Atomic "desugaring" is not fully correct as it does more
than remove sugar, but it is the right thing to do for most of the
things that we care about. We can change this back once we decide to
support atomic types more comprehensively.

Reviewers: teemperor, shafik

Subscribers: jfb, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71262