Use string.Builder

with 4000X clones of adbd, bp2build.Codegen time goes from 36s to 7s

Test: m bp2build and verify same BUILD.bazel files
Bug: 292281398
Change-Id: Ibd3b328a917fe5d1bdad67c67116da336f9033cb

bp2build/build_conversion.go

@@ -29,6 +29,7 @@ import (
 	"android/soong/bazel"
 	"android/soong/starlark_fmt"
 	"android/soong/ui/metrics/bp2build_metrics_proto"
+
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/bootstrap"
 	"github.com/google/blueprint/proptools"
@@ -94,16 +95,16 @@ func (targets BazelTargets) sort() {
 // statements (use LoadStatements for that), since the targets are usually not
 // adjacent to the load statements at the top of the BUILD file.
 func (targets BazelTargets) String() string {
-	var res string
+	var res strings.Builder
 	for i, target := range targets {
 		if target.ruleClass != "package" {
-			res += target.content
+			res.WriteString(target.content)
 		}
 		if i != len(targets)-1 {
-			res += "\n\n"
+			res.WriteString("\n\n")
 		}
 	}
-	return res
+	return res.String()
 }
 
 // LoadStatements return the string representation of the sorted and deduplicated