Use libcxx from Darwin SDKs when building LLVM and Swift

Those are present since Xcode 12.5, so we don't need to copy them
anymore from the toolchain

In this scenario, clean up any existing symlink in incremental builds to
avoid masking or causing errors in the future.

Addresses rdar://102387542

Author: edymtt

utils/build-script-impl

@@ -2822,26 +2822,37 @@ for host in "${ALL_HOSTS[@]}"; do
         # tree... but we are not building llvm with libcxx in tree when we build
         # swift. So we need to do configure's work here.
         if [[ "${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 [[ "$(uname -s)" == "Haiku" ]] ; then
-              HOST_CXX_HEADERS_DIR="/boot/system/develop/headers/c++"
-            elif [[ "${ANDROID_DATA}" ]] ; then
-              # This means we're building natively on Android in the Termux
-              # app, which supplies the $PREFIX variable.
-              HOST_CXX_HEADERS_DIR="$PREFIX/include/c++"
-            else # Linux
-              HOST_CXX_HEADERS_DIR="/usr/include/c++"
+                # We don't need this for Darwin since libcxx is present in SDKs present
+                # in Xcode 12.5 and onward (build-script requires Xcode 13.0 at a minimum),
+                # and clang knows how to find it there
+                # However, we should take care of removing the symlink
+                # laid down by a previous invocation, so to avoid failures
+                # finding c++ headers should the target folder become invalid
+                BUILT_CXX_INCLUDE_DIR="$llvm_build_dir/include/c++"
+                if [ -L "$BUILT_CXX_INCLUDE_DIR" ]; then
+                    echo "removing the symlink to system headers in the local clang build directory ($BUILT_CXX_INCLUDE_DIR)."
+                    call unlink "$BUILT_CXX_INCLUDE_DIR"
+                fi
+            else
+                # Find the location of the c++ header dir.
+                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
+                  # app, which supplies the $PREFIX variable.
+                  HOST_CXX_HEADERS_DIR="$PREFIX/include/c++"
+                else # Linux
+                  HOST_CXX_HEADERS_DIR="/usr/include/c++"
+                fi
+
+                # Find the path in which the local clang build is expecting to find
+                # the c++ header files.
+                BUILT_CXX_INCLUDE_DIR="$llvm_build_dir/include"
+
+                echo "symlinking the system headers ($HOST_CXX_HEADERS_DIR) into the local clang build directory ($BUILT_CXX_INCLUDE_DIR)."
+                call ln -s -f "$HOST_CXX_HEADERS_DIR" "$BUILT_CXX_INCLUDE_DIR"
             fi
-
-            # Find the path in which the local clang build is expecting to find
-            # the c++ header files.
-            BUILT_CXX_INCLUDE_DIR="$llvm_build_dir/include"
-
-            echo "symlinking the system headers ($HOST_CXX_HEADERS_DIR) into the local clang build directory ($BUILT_CXX_INCLUDE_DIR)."
-            call ln -s -f "$HOST_CXX_HEADERS_DIR" "$BUILT_CXX_INCLUDE_DIR"
         fi
 
         # Build.