Add compiler option that allows to pass arguments to Clang

It is useful for compiler exploration and tuning.

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

@@ -152,6 +152,7 @@ class K2Native : CLICompiler<K2NativeCompilerArguments>() {
                 put(DEBUG, arguments.debug)
                 put(LIGHT_DEBUG, arguments.lightDebug)
                 put(STATIC_FRAMEWORK, selectFrameworkType(configuration, arguments, outputKind))
+                put(OVERRIDE_CLANG_OPTIONS, arguments.clangOptions.toNonNullList())
 
                 put(PRINT_IR, arguments.printIr)
                 put(PRINT_IR_WITH_DESCRIPTORS, arguments.printIrWithDescriptors)

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

@@ -231,6 +231,9 @@ class K2NativeCompilerArguments : CommonCompilerArguments() {
     @Argument(value = "-Xobjc-generics", description = "Enable experimental generics support for framework header")
     var objcGenerics: Boolean = false
 
+    @Argument(value="-Xoverride-clang-options", valueDescription = "<arg1,arg2,...>", description = "Explicit list of Clang options")
+    var clangOptions: Array<String>? = null
+
     override fun configureAnalysisFlags(collector: MessageCollector): MutableMap<AnalysisFlag<*>, Any> =
             super.configureAnalysisFlags(collector).also {
                 val useExperimental = it[AnalysisFlags.useExperimental] as List<*>

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/BitcodeCompiler.kt

@@ -18,6 +18,9 @@ internal class BitcodeCompiler(val context: Context) {
     private val optimize = context.shouldOptimize()
     private val debug = context.config.debug
 
+    private val overrideClangOptions =
+            context.configuration.getList(KonanConfigKeys.OVERRIDE_CLANG_OPTIONS)
+
     private fun MutableList<String>.addNonEmpty(elements: List<String>) {
         addAll(elements.filter { it.isNotEmpty() })
     }
@@ -58,7 +61,8 @@ internal class BitcodeCompiler(val context: Context) {
             }
             else -> context.llvm.targetTriple
         }
-        val flags = mutableListOf<String>().apply {
+        val flags = overrideClangOptions.takeIf(List<String>::isNotEmpty)
+                ?: mutableListOf<String>().apply {
             addNonEmpty(configurables.clangFlags)
             addNonEmpty(listOf("-triple", targetTriple))
             if (configurables is ZephyrConfigurables) {

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/KonanConfigurationKeys.kt

@@ -84,6 +84,8 @@ class KonanConfigKeys {
                 = CompilerConfigurationKey.create("optimized compilation")
         val OUTPUT: CompilerConfigurationKey<String> 
                 = CompilerConfigurationKey.create("program or library name")
+        val OVERRIDE_CLANG_OPTIONS: CompilerConfigurationKey<List<String>>
+                = CompilerConfigurationKey.create("arguments for clang")
         val PRINT_BITCODE: CompilerConfigurationKey<Boolean> 
                 = CompilerConfigurationKey.create("print bitcode")
         val PRINT_DESCRIPTORS: CompilerConfigurationKey<Boolean>