go/parser: fix incorrect resolution of receiver type parameters

Declare receiver type parameters in the function scope, but don't
resolve them (for now), as ast.Object.Decl is not documented to hold
*ast.Idents. This avoids incorrect resolution of identifiers to names
outside the function scope.

Also make tracing and error reporting more consistent.

For golang/go#50956

Change-Id: I8cd61dd25f4c0f6b974221599b00e23d8da206a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/382247


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>