cmd/go: don't add generated SWIG C++ files to CompiledGoFiles

Also include SWIG C++ files in cgo hash.

For #28749
Fixes #37098

Change-Id: I6d912db2788200c2abdf328e382d4fbefda0a9ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/313131
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/work/exec.go

@@ -246,11 +246,11 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 	if p.Internal.ForceLibrary {
 		fmt.Fprintf(h, "forcelibrary\n")
 	}
-	if len(p.CgoFiles)+len(p.SwigFiles) > 0 {
+	if len(p.CgoFiles)+len(p.SwigFiles)+len(p.SwigCXXFiles) > 0 {
 		fmt.Fprintf(h, "cgo %q\n", b.toolID("cgo"))
 		cppflags, cflags, cxxflags, fflags, ldflags, _ := b.CFlags(p)
 		fmt.Fprintf(h, "CC=%q %q %q %q\n", b.ccExe(), cppflags, cflags, ldflags)
-		if len(p.CXXFiles)+len(p.SwigFiles) > 0 {
+		if len(p.CXXFiles)+len(p.SwigCXXFiles) > 0 {
 			fmt.Fprintf(h, "CXX=%q %q\n", b.cxxExe(), cxxflags)
 		}
 		if len(p.FFiles) > 0 {
@@ -654,6 +654,10 @@ OverlayLoop:
 		}
 
 		outGo, outObj, err := b.cgo(a, base.Tool("cgo"), objdir, pcCFLAGS, pcLDFLAGS, mkAbsFiles(a.Package.Dir, cgofiles), gccfiles, cxxfiles, a.Package.MFiles, a.Package.FFiles)
+
+		// The files in cxxfiles have now been handled by b.cgo.
+		cxxfiles = nil
+
 		if err != nil {
 			return err
 		}

src/cmd/go/testdata/script/list_swigcxx.txt

@@ -0,0 +1,27 @@
+# go list should not report SWIG-generated C++ files in CompiledGoFiles.
+
+[!exec:swig] skip
+[!exec:g++] skip
+
+# CompiledGoFiles should contain 4 files:
+#  a.go
+#  a.swigcxx.go
+#  _cgo_gotypes.go
+#  a.cgo1.go
+
+go list -f '{{.CompiledGoFiles}}' -compiled=true example/swig
+
+# These names we see here, other than a.go, will be from the build cache,
+# so we just count them.
+stdout a\.go
+stdout -count=3 $GOCACHE
+
+-- go.mod --
+module example
+
+go 1.16
+
+-- swig/a.go --
+package swig
+
+-- swig/a.swigcxx --