[K/N] Add option to save unoptimized bitcode

Add `-Xsave-llvm-ir` option that stores LLVM IR text into temporary
directory right after IrToBitcode translation. This is required for
FileCheck-based tests and just helpful during development.

kotlin-native/backend.native/cli.bc/src/org/jetbrains/kotlin/cli/bc/K2Native.kt

@@ -175,6 +175,7 @@ class K2Native : CLICompiler<K2NativeCompilerArguments>() {
                 arguments.manifestFile ?.let{ put(MANIFEST_FILE, it) }
                 arguments.runtimeFile ?.let{ put(RUNTIME_FILE, it) }
                 arguments.temporaryFilesDir?.let { put(TEMPORARY_FILES_DIR, it) }
+                put(SAVE_LLVM_IR, arguments.saveLlvmIr)
 
                 put(LIST_TARGETS, arguments.listTargets)
                 put(OPTIMIZATION, arguments.optimization)

kotlin-native/backend.native/cli.bc/src/org/jetbrains/kotlin/cli/bc/K2NativeCompilerArguments.kt

@@ -255,6 +255,9 @@ class K2NativeCompilerArguments : CommonCompilerArguments() {
     @Argument(value = "-Xtemporary-files-dir", deprecatedName = "--temporary_files_dir", valueDescription = "<path>", description = "Save temporary files to the given directory")
     var temporaryFilesDir: String? = null
 
+    @Argument(value = "-Xsave-llvm-ir", description = "Save result of Kotlin IR to LLVM IR translation to the temporary files directory.")
+    var saveLlvmIr: Boolean = false
+
     @Argument(value = "-Xverify-bitcode", deprecatedName = "--verify_bitcode", description = "Verify llvm bitcode after each method")
     var verifyBitCode: Boolean = false