WeakMap is an associative container of keys of type K to values of type std::weak_ptr<V>.
WeakMap's iterators will skip over elements where the value has no more remaining std::shared_ptr<V> references.

Used to automatically track live debug objects.

Bug: b/145351270
Change-Id: I87b9a9be4b83ff887510f292a4e03770ccc9eeaf
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38894Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>