[dev.typeparams] cmd/compile: start using sub-dictionary entries where needed

Added new struct instInfo for information about an instantiation (of a
generic function/method with gcshapes or concrete types). We use this to
remember the dictionary param node, the nodes where sub-dictionaries
need to be used, etc. The instInfo map replaces the Stencil map in
Package.

Added code to access sub-dictionary entries at the appropriate call
sites. We are currently still calculating the corresponding main
dictionary, even when we really only need a sub-dictionary. I'll clean
that up in a follow-up CL.

Added code to deal with "generic" closures (closures that reference some
generic variables/types). We decided that closures will share the same
dictionary as the containing function (accessing the dictionary via a
closure variable). So, the getGfInfo function now traverses all the
nodes of each closure in a function that it is analyzing, so that a
function's dictionary has all the entries needed for all its closures as
well. Also, the instInfo of a closure is largely shared with its
containing function. A good test for generic closures already exists
with orderedmap.go.

Other improvements:
 - Only create sub-dictionary entries when the function/method
   call/value or closure actually has type params in it. Added new test
   file subdict.go with an example where a generic method has an
   instantiated method call that does not depend not have type params.

Change-Id: I691b9dc024a89d2305fcf1d8ba8540e53c9d103f
Reviewed-on: https://go-review.googlesource.com/c/go/+/331516


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>