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.

compiler/incremental-compilation-impl/src/org/jetbrains/kotlin/incremental/IncrementalJvmCompilerRunner.kt

@@ -368,6 +368,7 @@ class IncrementalJvmCompilerRunner(
         val compiler = K2JVMCompiler()
         val freeArgsBackup = args.freeArgs.toList()
         args.freeArgs += sourcesToCompile.map { it.absolutePath }
+        args.allowNoSourceFiles = true
         val exitCode = compiler.exec(messageCollector, services, args)
         args.freeArgs = freeArgsBackup
         return exitCode