Simplify identifier creation in ConcatenatedModules

This was implemented in #5997 to fix some memory issues, but I think we
can make this a little more efficient. By using a single forEach instead
of a map, filter, and join, we avoid unnecessary array creations and
iterations.

lib/optimize/ConcatenatedModule.js

@@ -295,14 +295,12 @@ class ConcatenatedModule extends Module {
 	}
 
 	_createIdentifier() {
-		const orderedConcatenationListIdentifiers = this._orderedConcatenationList.map(info => {
-			switch(info.type) {
-				case "concatenated":
-					return info.module.identifier();
-			}
-		}).filter(Boolean).join(" ");
 		const hash = crypto.createHash("md5");
-		hash.update(orderedConcatenationListIdentifiers);
+		this._orderedConcatenationList.forEach(info => {
+			if(info.type === "concatenated") {
+				hash.update(info.module.identifier() + " ");
+			}
+		});
 		return this.rootModule.identifier() + " " + hash.digest("hex");
 	}