[Demangler] Handle invertible reqs for extensions in `findDeclContext`
If we didn't find an extension result, try again disregarding
invertible requirements since `demangleGenericSignature` won't
include them. This is just meant to be a quick low risk fix that we
can cherry-pick, the proper fix here is to delete all this logic and
just return the nominal along with the ABI module name to filter
lookup results.
rdar://165639044