cmd/compile: switch to final unified IR export format

Now that there's a native go/types importer for unified IR, the
compiler no longer needs to stay backwards compatible with old iexport
importers.

This CL also updates the go/types and go/internal/gcimporter tests to
expect that the unified IR importer sets the receiver parameter type
to the underlying Interface type, rather than the Named type. This is
a temporary workaround until we make a decision on #49906.

Notably, this makes `GOEXPERIMENT=unified go test` work on generics
code without requiring `-vet=off` (because previously cmd/vet was
relying on unified IR's backwards-compatible iexport data, which
omitted generic types).

Change-Id: Iac7a2346bb7a91e6690fb2978fb702fadae5559d
Reviewed-on: https://go-review.googlesource.com/c/go/+/386004
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -589,10 +589,6 @@ func (pr *pkgReader) objIdx(idx int, implicits, explicits []*types.Type) ir.Node
 		if pri, ok := objReader[sym]; ok {
 			return pri.pr.objIdx(pri.idx, nil, explicits)
 		}
-		if haveLegacyImports {
-			assert(len(explicits) == 0)
-			return typecheck.Resolve(ir.NewIdent(src.NoXPos, sym))
-		}
 		base.Fatalf("unresolved stub: %v", sym)
 	}
 
@@ -1972,12 +1968,6 @@ func InlineCall(call *ir.CallExpr, fn *ir.Func, inlIndex int) *ir.InlinedCallExp
 
 	pri, ok := bodyReader[fn]
 	if !ok {
-		// Assume it's an imported function or something that we don't
-		// have access to in quirks mode.
-		if haveLegacyImports {
-			return nil
-		}
-
 		base.FatalfAt(call.Pos(), "missing function body for call to %v", fn)
 	}