cmd/compile/internal/types2: use an identifier map rather than isubst for recv type params

This is a port of CL 354643 from go/types to types2 with adjustments:
- use of syntax rather than go/ast package as needed
- adjustments due to the different code for type parameter declarations
- rename of Checker.rparamMap to Checker.recvTParamMap, which seems clearer

Change-Id: I5311a0c05a13c6b87ea1422b250b90c3d05c5dce
Reviewed-on: https://go-review.googlesource.com/c/go/+/354693


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>