Treat '.ccm', '.cxxm', and '.c++m' files as C++ sources

These extensions are used by convention for C++ module interface units
with Clang.

For now, do not add any tests using these extensions.
Very few compilers recognize them as C++ sources.
Tests can be added later as part of C++ modules support.

Help/release/dev/cxx-module-extensions.rst

@@ -0,0 +1,5 @@
+cxx-module-extensions
+---------------------
+
+* Source file extensions ``.ccm``, ``.cxxm``, or ``.c++m`` are now
+  treated as C++.