cmd/compile: don't create 2 Sym's and 2 Node's for every string

For every string constant the compiler was creating 2 Sym's and 2
Node's.  It would never refer to them again, but would keep them alive
in gostringpkg.  This changes the code to just use obj.LSym's instead.

When compiling x/tools/go/types, this yields about a 15% reduction in
the number of calls to newname and a 3% reduction in the total number of
Node objects.  Unfortunately I couldn't see any change in compile time,
but reducing memory usage is desirable anyhow.

Passes toolstash -cmp.

Change-Id: I24f1cb1e6cff0a3afba4ca66f7166874917a036b
Reviewed-on: https://go-review.googlesource.com/20792
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/link.go

@@ -343,6 +343,12 @@ type LSym struct {
 	R      []Reloc
 }
 
+// The compiler needs LSym to satisfy fmt.Stringer, because it stores
+// an LSym in ssa.ExternSymbol.
+func (s *LSym) String() string {
+	return s.Name
+}
+
 type Pcln struct {
 	Pcsp        Pcdata
 	Pcfile      Pcdata