[dev.typeparams] cmd/compile: enable generics syntax with -lang=go1.18

We already use -lang=go1.18 to control the types2 type checker
behavior. This CL does the same for the parser.

Also, disable an assertion in the unified IR linker that depended on
the -G flag. This assertion was more useful during initial
bootstrapping of that code, but it's less critical now.

With these two changes, "GOEXPERIMENT=unified ./make.bash" is enough
to get a fully functional generics-enabled toolchain. There's no need
to continue specifying custom compiler flags later on.

Change-Id: I7766381926f3bb17eee2e5fcc182a38a39e937e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/332373


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>