[clang][libclang]Check auto type for type constraints (#172472)
the "auto" keyword was visited by the default visitor only, making any
type constraints unavailable through the libclang API.
Add an explicit visitor for AutoType, check for constraints, and visit
those.
Fixes issue #166580
I modeled the visiting after
https://github.com/llvm/llvm-project/blob/main/clang/tools/libclang/CIndex.cpp#L1346
, and the tests after what I found in
https://github.com/llvm/llvm-project/blob/main/clang/test/Index/index-concepts.cpp
. Hopefully it fits.