[lldb] Centralize desugaring of decltype-like types in ClangASTContext

Summary:
These types were handled in some places, but not others. This resulted
in (for example) not being able to display members of structs whose
types were defined using these constructs.

Using getLocallyUnqualifiedSingleStepDesugaredType for these types is
not fully equivalent, as it will only desugar them if the types are not
instantiation-dependent, whereas previously we did that unconditionally.

It's not clear to me which behavior is correct here, but the test suite
does not seem to care either way.

Reviewers: teemperor, shafik

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71405