[dev.typeparams] Parse a generic type arg for generic function call

Will now run "go tool compile -G=2 -W=2" on a simple generic function
with one type parameter and a call to that function with one explicit
type argument. Next change will handle multiple type arguments.

Change-Id: Ia7d17ea2a02bf99bd50e673ac80ae4aad4c48440
Reviewed-on: https://go-review.googlesource.com/c/go/+/288432


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>