cmd/compile: add support for alternative comparable semantics

Add the experimental compiler flag -altcomparable. If set, the
compiler uses alternative comparable semantics: any ordinary
(non-type parameter) interface implements the comparable
constraint.

This permits experimenting with this alternative semantics
akin to what is proposed in #52509.

For #52509.

Change-Id: I64192eee6f2a550eeb50de011079f2f0b994cf94
Reviewed-on: https://go-review.googlesource.com/c/go/+/444636
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/base/flag.go

@@ -123,6 +123,7 @@ type CmdFlags struct {
 	TraceProfile       string       "help:\"write an execution trace to `file`\""
 	TrimPath           string       "help:\"remove `prefix` from recorded source file paths\""
 	WB                 bool         "help:\"enable write barrier\""                    // TODO: remove
+	AltComparable      bool         "help:\"enable alternative comparable semantics\"" // experiment - remove eventually
 
 	// Configuration derived from flags; not a flag itself.
 	Cfg struct {

src/cmd/compile/internal/noder/irgen.go

@@ -57,6 +57,7 @@ func checkFiles(noders []*noder) (posMap, *types2.Package, *types2.Info) {
 		},
 		Importer:               &importer,
 		Sizes:                  &gcSizes{},
+		AltComparableSemantics: base.Flag.AltComparable, // experiment - remove eventually
 	}
 	info := &types2.Info{
 		StoreTypesInSyntax: true,