Fix handling -Xclang arguments (#11173)

Fixes: #11172

Author: sbc100

emcc.py

@@ -989,7 +989,7 @@ def add_link_flag(i, f):
                  '-isysroot', '-imultilib', '-A', '-isystem', '-iquote',
                  '-install_name', '-compatibility_version',
                  '-current_version', '-I', '-L', '-include-pch',
-                 '-Xlinker'):
+                 '-Xlinker', '-Xclang'):
         skip = True
 
       if options.expand_symlinks and os.path.islink(arg) and get_file_suffix(os.path.realpath(arg)) in SOURCE_ENDINGS + OBJECT_FILE_ENDINGS + DYNAMICLIB_ENDINGS + ASSEMBLY_ENDINGS + HEADER_ENDINGS:

tests/test_other.py

@@ -10854,3 +10854,7 @@ def test_linker_version(self):
   def test_chained_js_error_diagnostics(self):
     err = self.expect_fail([PYTHON, EMCC, path_from_root('tests', 'test_chained_js_error_diagnostics.c'), '--js-library', path_from_root('tests', 'test_chained_js_error_diagnostics.js')])
     self.assertContained("error: undefined symbol: nonexistent_function (referenced by bar__deps: ['nonexistent_function'], referenced by foo__deps: ['bar'], referenced by top-level compiled C/C++ code)", err)
+
+  def test_xclang_flag(self):
+    create_test_file('foo.h', ' ')
+    run_process([PYTHON, EMCC, '-c', '-o', 'out.o', '-Xclang', '-include', '-Xclang', 'foo.h', path_from_root('tests', 'hello_world.c')])