[clang] Template Specialization Resugaring - TypeDecl
This is the introductory patch for a larger work which
intends to solve the long standing C++ issue with losing
type sugar when acessing template specializations.
The well known example here is specializing a template
with `std::string`, but getting diagnostics related to
`std::basic_string<char>` instead.
This implements a transform which, upon member access,
propagates type sugar from the naming context into the
accessed entity.
It also implements a single use of this transform,
resugaring access to TypeDecls.
For more details and discussion see:
https://discourse.llvm.org/t/rfc-improving-diagnostics-with-template-specialization-resugaring/64294
This is ready for review, although maybe not finished and
there is some more stuff that could be done either here
or in follow ups.
* Its worth exploring if a global resugaring cache is
worthwhile, besides the current operational cache.
A global cache would be more expensive to index, so there
is a tradeoff, and maybe should be used of the whole
result of the operation, while keeping the existing
cache for sub-results.
* It would be ideal if the transform could live in ASTContext
instead of Sema. There are a few dependencies that would
have to be tackled.
* Template arguments deduced for partial specializations.
* Some kinds of type adjustments currently require Sema.
Differential Revision: https://reviews.llvm.org/D127695
