[dev.typeparams] cmd/compile: better Call constructor

Historically, it's been tedious to create and typecheck ir.OCALL
nodes, except by handing them off entirely to typecheck. This is
because typecheck needed context on whether the call is an expression
or statement, and to set flags like Func.ClosureCalled and
CallExpr.Use.

However, those flags have now been removed entirely by recent CLs, so
we can instead just provide a better typecheck.Call function for
constructing and typechecking arbitrary call nodes. Notably, this
simplifies things for unified IR, which can now incrementally
typecheck call expressions as it goes without worrying about context.

Change-Id: Icbdc55c3bd8be84a242323bc45006f9dec09fdcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/332692


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