<map>, <unordered_map>: Allow MapLike<K, V&> again

[StephanTLavavej]

One of the changes that I pushed to #2436 was damaging - while map-like types can't tolerate references as key types, using references as mapped types appears to be permitted (by the Standard, libstdc++, and libc++), and there are 4 projects across vcpkg and Real World Code that are doing this.

Fortunately, we can fix this before changes stop flowing into VS 2022 17.7 Preview 2, so users will never be affected. I'm removing my bad static_asserts, and adding test coverage to minimally verify that this scenario compiles.

(I suppose we could instead assert that the mapped types aren't function types or void types, but I'd rather just drop it entirely. Such usage is extremely unlikely and will still be rejected, just with lengthy diagnostics.)

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[cjdb]

using references as mapped types appears to be permitted (by the Standard

[associative.reqmts.general]/8 says that the rules applying to value_type instead apply to key_type and mapped_type for our map types. If references are allowed for mapped_type, doesn't that imply they're also allowed for key_type and other containers' value_type?

[CaseyCarter]

I think the cross-references in [associative.reqmts.general] are swapped; it doesn't make sense to apply the value_type requirements from the allocator-aware requirements to key_type and mapped_type. cplusplus/draft#7249 tries to fix them.