[dev.typeparams] cmd/compile/internal/types2: convert untyped arguments to delete

For the predeclared "delete" function, types2 was checking that the
second argument was assignable to the map's key type, but not actually
updating the Types map as appropriate. So this could leave untyped
constants in the AST.

The error "cannot convert" is somewhat less precise than the previous
"not assignable" error, but it's consistent with how types2 reports
other erroneous assignments of untyped constants.

Change-Id: Ic3ca3a3611ad0e4646c050e93088cdf992234e5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/285059


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>