Use libcxx from Darwin SDKs when building LLVM and Swift

We don't need to copy them anymore from the toolchain

Addresses rdar://102387542

Author: edymtt

utils/build-script-impl

@@ -2821,12 +2821,10 @@ for host in "${ALL_HOSTS[@]}"; do
         # place during the cmake step of LLVM's build when libcxx is in
         # tree... but we are not building llvm with libcxx in tree when we build
         # swift. So we need to do configure's work here.
-        if [[ "${product}" == "llvm" ]]; then
-            # Find the location of the c++ header dir.
-            if [[ "$(uname -s)" == "Darwin" ]] ; then
-              HOST_CXX_DIR=$(dirname "${HOST_CXX}")
-              HOST_CXX_HEADERS_DIR="$HOST_CXX_DIR/../../usr/include/c++"
-            elif [[ "$(uname -s)" == "Haiku" ]] ; then
+        # We don't need this for Darwin since libcxx is present in the SDKs already,
+        # and clang knows how to find it there
+        if [[ "${product}" == "llvm" && "$(uname -s)" != "Darwin" ]]; then
+            if [[ "$(uname -s)" == "Haiku" ]] ; then
               HOST_CXX_HEADERS_DIR="/boot/system/develop/headers/c++"
             elif [[ "${ANDROID_DATA}" ]] ; then
               # This means we're building natively on Android in the Termux