cmd/compile: new absdiff.go test, fix problem with g.curDecl

Added a new absdiff2.go test case, which works fully without using a
typeparam on the right-hand-side of a type declaration (which is
disallowed). Fixed an issue that the test revealed, which is that we
need to set g.curDecl properly for the "later" functions which are
deferred until after all declarations are initially processed. Also,
g.curDecl may be non-nil in typeDecl for local type declaration. So, we
adjust the associate assertion, and save/restore g.curDecl
appropriately.

Fixes #50790

Change-Id: Ieed76a7ad0a83bccb99cbad4bf98a7bfafbcbbd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/380594


Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>