[K/N] Add xcodeOverride to Xcode in native-utils ^KTI-1553

- findCurrent() is used by the build of Kotlin/Native compiler itself.
  It happens indirectly, via AppleConfigurables.
- CurrentXcode invokes xcrun command line tool in its implementation.
- Configuration cache prohibits calling external commands

All three factors combined make K/N compiler build incompatible with CC.

By using xcodeOverride, K/N compiler build can provide its own Xcode
instance avoiding calling external commands when it's not allowed by CC.

native/utils/src/org/jetbrains/kotlin/konan/target/Xcode.kt

@@ -83,7 +83,11 @@ interface Xcode {
         val current: Xcode
             get() = findCurrent()
 
-        fun findCurrent(): Xcode = CurrentXcode()
+        var xcodeOverride: Xcode? = null
+
+        fun findCurrent(): Xcode = xcodeOverride ?: defaultCurrent()
+
+        fun defaultCurrent(): Xcode = CurrentXcode()
     }
 }