[K/N] Fix CC in Xcode usage ^KTI-1553

- Perform all calls to xcrun in XcodeValueSource.
- Use the fully computed Xcode instance as the current Xcode instance.
- Hide all of this inside a Gradle plugin and apply it to kotlin-native
  project.

kotlin-native/build.gradle

@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.cpp.CompilationDatabasePlugin
 import org.jetbrains.kotlin.cpp.CppUsage
 import org.jetbrains.kotlin.cpp.GitClangFormatPlugin
 import org.jetbrains.kotlin.CompareDistributionSignatures
+import org.jetbrains.kotlin.xcode.XcodeOverridePlugin
 import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
 import org.apache.tools.ant.filters.ReplaceTokens
 import org.jetbrains.kotlin.UtilsKt
@@ -44,6 +45,8 @@ buildscript {
 
 defaultTasks 'clean', 'dist'
 
+apply plugin: XcodeOverridePlugin
+
 if (PlatformInfo.isMac()) {
     PlatformInfo.checkXcodeVersion(project)
 }