x/tools/gopls: missing references to methods on generic types

[findleyr]

Repro:

• clone https://github.com/hashicorp/terraform-provider-aws
• open ./internal/types/option.go
• find references on methods, e.g. (Option[T]).MustUnwrap; gopls@v0.12.0 finds no references, but v0.11.0 does

CC @adonovan

[adonovan]

The root cause is failure of objectpath.For for the MustUnwrap method object during index construction (xrefs.Index).

[gopherbot]

Change https://go.dev/cl/506939 mentions this issue: gopls/internal/lsp/source/xrefs: include refs to methods of generics