cmd/compile/internal/types2: combine all type inference in a single function

Rather than splitting up type inference into function argument
and constraint type inference, provide a single Checker.infer
that accepts type parameters, type arguments, value parameters,
and value arguments, if any. Checker.infer returns the completed
list of type arguments, or nil.

Updated (and simplified) call sites.

Change-Id: I9200a44b9c4ab7f2d21eed824abfffaab68ff766
Reviewed-on: https://go-review.googlesource.com/c/go/+/306170


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>