Speed up SettingsBuilder (#1221)

* Speed up SettingsBuilder

It's unnecessary to build up a whole grouped dictionary only to check
if all platforms are identical and then immediately discard the
dictionary.

Instead we can check if all targets match the first platform, which
avoids creating a new dictionary but also allows bailing early as soon
as a non-matching platform is found.

Generating a large project (36MB json spec) on an M1 Max machine leads
to a ~6% total speedup: 28.48s vs 30.07s.

* Add changelog entry

Sources/XcodeGenKit/SettingsBuilder.swift

@@ -11,12 +11,10 @@ extension Project {
         var buildSettings: BuildSettings = [:]
 
         // set project SDKROOT is a single platform
-        if targets.count > 0 {
-            let platforms = Dictionary(grouping: targets) { $0.platform }
-            if platforms.count == 1 {
-                let platform = platforms.first!.key
-                buildSettings["SDKROOT"] = platform.sdkRoot
-            }
+        if let firstPlatform = targets.first?.platform,
+           targets.allSatisfy({ $0.platform == firstPlatform })
+        {
+            buildSettings["SDKROOT"] = firstPlatform.sdkRoot
         }
 
         if let type = config.type, options.settingPresets.applyProject {
@@ -31,7 +29,7 @@ extension Project {
             }
         }
 
-        // Prevent setting presets from overrwriting settings in project xcconfig files
+        // Prevent setting presets from overwriting settings in project xcconfig files
         if let configPath = configFiles[config.name] {
             buildSettings = removeConfigFileSettings(from: buildSettings, configPath: configPath)
         }