cmd/compile: fix case where g.curDecl should be saved/restored

When we set g.curDecl for the type params created during fillinMethods
for an instantiated type, we need to save/restore its value, because
fillinMethods() may be called while processing a typeDecl. We want the
value of g.curDecl to continue to be correct for type params created in
the typeDecl. Because of ordering issues, not restoring g.curDecl
happens to cause problems (which don't always show up visibly) exactly
when a type param is not actually used in a type declaration.

Cleared g.curDecl to "" at the later points in typeDecl() and
funcDecl(). This allows adding asserts that g.curDecl is always empty
("") when we set it in typeDecl() and funcDecl(), and always non-empty
when we use it in typ0().

Fixes #49893

Change-Id: Ic2fb1df791585bd257f2b86ffaae0453c31705c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/368454


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>