[Kotlin/Native] Add a workaround for Xcode 12.5

SDKs from Xcode 12.5 contain C++ stdlib and it
breaks compilation of C++ parts of compiler.
Mitigate this problem by explicitly specifying
path to C++ stdlib.

kotlin-native/shared/src/main/kotlin/org/jetbrains/kotlin/konan/target/ClangArgs.kt

@@ -50,7 +50,6 @@ class ClangArgs(private val configurables: Configurables) : Configurables by con
             add(listOf("--gcc-toolchain=${configurables.absoluteGccToolchain}"))
         }
         if (configurables is AppleConfigurables) {
-            add(listOf("-stdlib=libc++"))
             val osVersionMin = when (target) {
                 // Here we workaround Clang 8 limitation: macOS major version should be 10.
                 // So we compile runtime with version 10.16 and then override version in BitcodeCompiler.
@@ -157,10 +156,28 @@ class ClangArgs(private val configurables: Configurables) : Configurables by con
 
     val hostCompilerArgsForJni = listOf("", HostManager.jniHostPlatformIncludeDir).map { "-I$jdkDir/include/$it" }.toTypedArray()
 
-    val clangArgs = (commonClangArgs + specificClangArgs).toTypedArray()
+    /**
+     * Clang args for Objectice-C and plain C compilation.
+     */
+    val clangArgs: Array<String> = (commonClangArgs + specificClangArgs).toTypedArray()
+
+    /**
+     * Clang args for C++ compilation.
+     */
+    val clangXXArgs: Array<String> = clangArgs + when (configurables) {
+        is AppleConfigurables -> arrayOf(
+                "-stdlib=libc++",
+                // Starting from Xcode 12.5, platform SDKs contain C++ stdlib.
+                // It results in two c++ stdlib in search path (one from LLVM, another from SDK).
+                // We workaround this problem by explicitly specifying path to stdlib.
+                // TODO: Revise after LLVM update.
+                "-nostdinc++", "-isystem", "$absoluteLlvmHome/include/c++/v1"
+        )
+        else -> emptyArray()
+    }
 
     val clangArgsForKonanSources =
-            clangArgs + clangArgsSpecificForKonanSources
+            clangXXArgs + clangArgsSpecificForKonanSources
 
     val targetLibclangArgs: List<String> =
             // libclang works not exactly the same way as the clang binary and
@@ -173,7 +190,7 @@ class ClangArgs(private val configurables: Configurables) : Configurables by con
             = listOf("${absoluteLlvmHome}/bin/clang") + clangArgs
 
     private val targetClangXXCmd
-            = listOf("${absoluteLlvmHome}/bin/clang++") + clangArgs
+            = listOf("${absoluteLlvmHome}/bin/clang++") + clangXXArgs
 
     private val targetArCmd
             = listOf("${absoluteLlvmHome}/bin/llvm-ar")