[dev.typeparams] cmd/compile: enable parsing of generic code with new -G flag

Providing the -G flag instructs the compiler to accept type parameters.
For now, the compiler only parses such files and then exits.

Added a new test directory (test/typeparam) and initial test case.

Port from dev.go2go branch.

Change-Id: Ic11e33a9d5f012f8def0bdae205043659562ac73
Reviewed-on: https://go-review.googlesource.com/c/go/+/261660


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>