cmd/compile/internal/types2: make ptrRecv a method hasPtrRecv of Func
Reading the Func.hasPtrRecv field directly (without consulting a
possibly existing signature) caused some issues in an earlier CL.
The function ptrRecv (in lookup.go) does the right thing but is
not easily discoverable.
Rename ptrRecv to hasPtrRecv and make it a method of Func; rename
Func.hasPtrRecv field to Func.hasPtrRecv_ to avoid name collisions.
Make it clear in field comment that it must not be read through the
hasPtrRecv method.
Change-Id: Ida9856c4789e499538eb34377be781655958fd5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/351310
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>