[dev.typeparams] Get dictionaryCapture.go working.

METHVALUE in a generic function (that is not called) was not causing
buildClosure() to be called and therefore not using dictionaries. Also,
had to add an extra check to make sure that if we have a FUNCINST
node above a METHVALUE, we only call buildClosure once.

Change-Id: I49756152fc343e5ac1c449e697960fc2a0f482ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/336429


Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>