[dev.link] cmd/link: free compressed symbols after dwarfcompress

Once the symbol is compressed, we will not need the uncompressed
symbol content. Free its memory.

Linking cmd/compile,

name               old live-B     new live-B     delta
Dwarfcompress_GC      42.7M ± 0%     37.9M ± 0%   -11.31%  (p=0.008 n=5+5)

Change-Id: Ib6cc73832946d158ff4f5b4f31be9c35ba7cf103
Reviewed-on: https://go-review.googlesource.com/c/go/+/230859
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/link/internal/loader/loader.go

@@ -2054,7 +2054,6 @@ func (l *Loader) LoadFull(arch *sys.Arch, syms *sym.Symbols, needReloc bool) {
 	// be copied in a later loop).
 	toConvert := make([]Sym, 0, len(l.payloads))
 	for _, i := range l.extReader.syms {
-		sname := l.RawSymName(i)
 		if !l.attrReachable.Has(i) {
 			continue
 		}
@@ -2065,7 +2064,7 @@ func (l *Loader) LoadFull(arch *sys.Arch, syms *sym.Symbols, needReloc bool) {
 		// outer/sub processing below. Note that once we do this,
 		// we'll need to get at the payload for a symbol with direct
 		// reference to l.payloads[] as opposed to calling l.getPayload().
-		s := l.allocSym(sname, 0)
+		s := l.allocSym(pp.name, 0)
 		l.installSym(i, s)
 		toConvert = append(toConvert, i)
 	}
@@ -2655,6 +2654,13 @@ func (l *Loader) CreateStaticSym(name string) Sym {
 	return l.newExtSym(name, l.anonVersion)
 }
 
+func (l *Loader) FreeSym(i Sym) {
+	if l.IsExternal(i) {
+		pp := l.getPayload(i)
+		*pp = extSymPayload{}
+	}
+}
+
 func loadObjFull(l *Loader, r *oReader, needReloc bool) {
 	for i, n := 0, r.NSym()+r.NNonpkgdef(); i < n; i++ {
 		// A symbol may be a dup or overwritten. In this case, its