go/internal/gccgoimporter: support notinheap annotation

Port https://golang.org/cl/265702 from the main Go repo.

Original CL description:
    The gofrontend has started emitting a notinheap annotation for types
    marked go:notinheap.

For golang/go#41761

Change-Id: Ic14ffda4b0c3eef850ad85dbf9af755283a5196b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/265718
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

go/internal/gccgoimporter/importer_test.go

@@ -100,6 +100,7 @@ var importerTests = [...]importerTest{
 	{pkgpath: "issue30628", name: "Apple", want: "type Apple struct{hey sync.RWMutex; x int; RQ [517]struct{Count uintptr; NumBytes uintptr; Last uintptr}}"},
 	{pkgpath: "issue31540", name: "S", gccgoVersion: 7, want: "type S struct{b int; map[Y]Z}"},
 	{pkgpath: "issue34182", name: "T1", want: "type T1 struct{f *T2}"},
+	{pkgpath: "notinheap", name: "S", want: "type S struct{}"},
 }
 
 func TestGoxImporter(t *testing.T) {

go/internal/gccgoimporter/parser.go

@@ -521,6 +521,13 @@ func (p *parser) parseNamedType(nlist []interface{}) types.Type {
 		p.errorf("%v has nil type", obj)
 	}
 
+	if p.tok == scanner.Ident && p.lit == "notinheap" {
+		p.next()
+		// The go/types package has no way of recording that
+		// this type is marked notinheap. Presumably no user
+		// of this package actually cares.
+	}
+
 	// type alias
 	if p.tok == '=' {
 		p.next()

go/internal/gccgoimporter/testdata/notinheap.go

@@ -0,0 +1,4 @@
+package notinheap
+
+//go:notinheap
+type S struct{}

go/internal/gccgoimporter/testdata/notinheap.gox

@@ -0,0 +1,7 @@
+v3;
+package notinheap
+pkgpath notinheap
+init notinheap ~notinheap
+types 3 2 30 18
+type 1 "S" notinheap <type 2>
+type 2 struct { }