[K/N] Add -Xllvm-variant` compiler option

We need a handle that allows user to pick `dev` variant instead of
`user` one just in case.
Additionally, I added a possibility to provide a path to an arbitrary
LLVM distribution because it is useful for development purposes.

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

@@ -353,6 +353,20 @@ class K2Native : CLICompiler<K2NativeCompilerArguments>() {
                 })
 
                 arguments.externalDependencies?.let { put(EXTERNAL_DEPENDENCIES, it) }
+                putIfNotNull(LLVM_VARIANT, when (val variant = arguments.llvmVariant) {
+                    "user" -> LlvmVariant.User
+                    "dev" -> LlvmVariant.Dev
+                    null -> null
+                    else -> {
+                        val file = File(variant)
+                        if (!file.exists) {
+                            configuration.report(ERROR, "`-Xllvm-variant` should be `user`, `dev` or an absolute path. Got: $variant")
+                            null
+                        } else {
+                            LlvmVariant.Custom(file)
+                        }
+                    }
+                })
             }
         }
     }

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

@@ -337,6 +337,13 @@ class K2NativeCompilerArguments : CommonCompilerArguments() {
     )
     var workerExceptionHandling: String? = null
 
+    @Argument(
+            value = "-Xllvm-variant",
+            valueDescription = "{dev|user}",
+            description = "Choose LLVM distribution which will be used during compilation."
+    )
+    var llvmVariant: String? = null
+
     override fun configureAnalysisFlags(collector: MessageCollector, languageVersion: LanguageVersion): MutableMap<AnalysisFlag<*>, Any> =
             super.configureAnalysisFlags(collector, languageVersion).also {
                 val useExperimental = it[AnalysisFlags.useExperimental] as List<*>