go/types: record pointer and parentheses in receiver expressions

CL 594740 rewrote type checking of method receiver types. Because that
CL takes apart receivers "manually" rather than using the regular code
for type checking type expressions, pointer and parenthesized receiver
type expressions were not recorded anymore.

Adjust the code that typechecks method receivers to a) use ordinary
type expression checking for non-generic receivers, and b) to record
a missing pointer and any intermediate parenthesized expressions in
case of a generic receiver.

Add many extra tests verifying that the correct types for parenthesized
and pointer type expressions are recorded in various source positions.

Note that the parser used by the compiler and types2 doesn't encode
unnecessary parentheses in type expressions in its syntax tree.
As a result, the tests that explicitly test parentheses don't work
in types2 and are commented out.

This CL adds code (disabled by default) to the parser to encode
parentheses in type expressions in the syntax tree. When enabled,
the commented out types2 tests pass like in go/types.

Fixes #68639.
For #51343.

Change-Id: Icf3d6c76f7540ee53e229660be8d78bb25380539
Reviewed-on: https://go-review.googlesource.com/c/go/+/601657
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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