Previously we would mangle a single ErrorType as
the type for the element, but that would fail to
demangle since we expect a function type. Use the
same logic as AbstractFunctionDecl, mangling a
function type of ErrorType. While here, also
handle SubscriptDecls and check for error types
rather than `isInvalid()`.
rdar://129065620
Properties can also be specified in a protocol/overridden by subclasses,
so they should also be classed as "dynamic" in these cases.
Removed receiver USRs when *not* dynamic, since it's not used for
anything in that case and should be equivalent to the container anyway.
Resolves rdar://92882348.
