[dev.typeparams] merge master (2f0da6d9) into dev.typeparams

This was a mostly clean merge, with the exception of codereview.cfg and
changes in src/go/types.

codereview.cfg for dev.typeparams is preserved in this CL. It should be
deleted before merging back to master.

The go/types changes were merged manually. For the most part this
involved taking the union of patches, with the following exceptions:
 + declInfo.aliasPos is removed, as it is not necessary in
   dev.typeparams where we have access to the full TypeSpec.
 + Checker.overflow is updated to use the asBasic converter.
 + A TODO is added to errorcodes.go to ensure that go1.16 error codes
   are preserved.

Change-Id: If9595196852e2163e27a9478df1e7b2c3704947d