Use libcxx from Darwin SDKs when building LLVM and Swift (LLVM product)

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/swift_build_support/swift_build_support/products/llvm.py

@@ -363,13 +363,24 @@ def _handle_cxx_headers(self, host_target, platform):
         # 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.
-        host_cxx_headers_dir = None
         if system() == 'Darwin':
-            host_cxx_dir = os.path.dirname(self.toolchain.cxx)
-            host_cxx_headers_dir = os.path.join(host_cxx_dir, os.pardir, os.pardir,
-                                                '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
+            cxx_include_symlink = os.path.join(self.build_dir, 'include', 'c++')
+            if os.path.islink(cxx_include_symlink):
+                print('removing the symlink to system headers in the local '
+                      f'clang build directory {cxx_include_symlink} .',
+                      flush=True)
+                shell.remove(cxx_include_symlink)
+
+            return
 
-        elif system() == 'Haiku':
+        host_cxx_headers_dir = None
+        if system() == 'Haiku':
             host_cxx_headers_dir = '/boot/system/develop/headers/c++'
 
         # This means we're building natively on Android in the Termux
@@ -395,7 +406,7 @@ def _handle_cxx_headers(self, host_target, platform):
             os.makedirs(built_cxx_include_dir)
         print('symlinking the system headers ({}) into the local '
               'clang build directory ({}).'.format(
-                  host_cxx_headers_dir, built_cxx_include_dir))
+                  host_cxx_headers_dir, built_cxx_include_dir), flush=True)
         shell.call(['ln', '-s', '-f', host_cxx_headers_dir, built_cxx_include_dir])
 
     def should_test(self, host_target):

utils/swift_build_support/swift_build_support/shell.py

@@ -202,6 +202,15 @@ def symlink(source, dest, dry_run=None, echo=True):
     os.symlink(source, dest)
 
 
+def remove(path, dry_run=None, echo=True):
+    dry_run = _coerce_dry_run(dry_run)
+    if dry_run or echo:
+        _echo_command(dry_run, ['rm', path])
+    if dry_run:
+        return
+    os.remove(path)
+
+
 # Initialized later
 lock = None