Add argument to allow running compiler without sources

`-Xbuild-file` argument allows the compiler to run without
passing any Kotlin source file in arguments.
We have been using this property in
Kotlin Gradle plugin for a few important cases:
1. incremental compilation (to update caches when there are only removed files);
2. for KAPT (Kotlin sources don't make sense in context
of running APs).

We want to stop using `-Xbuild-file` in Kotlin Gradle plugin,
and avoid breaking the Gradle plugin or IC in other build-systems.

This change adds an argument to explicitly run
the compiler without specifying any Kotlin source file.

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -405,6 +405,7 @@ open class KotlinCompile : AbstractKotlinCompile<K2JVMCompilerArguments>(), Kotl
 
         if (defaultsOnly) return
 
+        args.allowNoSourceFiles = true
         args.classpathAsList = compileClasspath.toList()
         args.destinationAsFile = destinationDir
         parentKotlinOptionsImpl?.updateArguments(args)