go/types, types2: ensure signatures are instantiated if all type args

go/types, types2: ensure signatures are instantiated if all type args
are provided

Improve the accuracy of recorded types and instances for function calls,
by instantiating their signature before checking arguments if all type
arguments are provided. This avoids a problem where fully instantiated
function signatures are are not recorded as such following an error
checking their arguments.

Fixes golang/go#51803

Change-Id: Iec4cbd219a2cd19bb1bcf2a5c4019f556e4304b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/451436
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>