go/parser: allow eliding interface in constraint literals

This is a port of CL 353133 from cmd/compile/internal/syntax, with
significant adjustments for the mechanics of go/parser.

Some additional cleanup is made along the way: parseParameterList can
call parseParamDecl without indirection, and the tparams argument is
redundant with the closing token. Also, the error that "all type
parameters must be named" is positioned on the first unnamed type
parameter.

Error recovery in go/parser is notably worse here than the compiler
parser, so the test data had to be adjusted to synchronize positions.
Fixing this error recovery will have to wait for a later CL.

As with the compiler changes, these changes are guarded behind a flag so
that they may be easily removed if #48424 is not accepted.

For #48424

Change-Id: If87925d246f36aaab11a95442f75f659462d4286
Reviewed-on: https://go-review.googlesource.com/c/go/+/354870


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