Revert "Revert "Merge to upstream r304942.""

This reverts commit 38a0d5af7e.

Test: make checkbuild
Test: ./run_tests.py
Test: ./run_tests.py --bitness 64

utils/libcxx/compiler.py

@@ -24,6 +24,7 @@ class CXXCompiler(object):
                  modules_flags=None, use_modules=False,
                  use_ccache=False, use_warnings=False, compile_env=None,
                  cxx_type=None, cxx_version=None):
+        self.source_lang = 'c++'
         self.path = path
         self.flags = list(flags or [])
         self.compile_flags = list(compile_flags or [])
@@ -108,7 +109,7 @@ class CXXCompiler(object):
         if out is not None:
             cmd += ['-o', out]
         if input_is_cxx:
-            cmd += ['-x', 'c++']
+            cmd += ['-x', self.source_lang]
         if isinstance(source_files, list):
             cmd += source_files
         elif isinstance(source_files, str):