Add support for C++ modules

[perryjiang]

Clang: https://clang.llvm.org/docs/StandardCPlusPlusModules.html

The file name of an importable module unit should end with .cppm (or .ccm, .cxxm, .c++m). The file name of a module implementation unit should end with .cpp (or .cc, .cxx, .c++).

I added support for the c++ file extension too.

Also, clang-format: https://github.com/llvm/llvm-project/blob/676629598595605dd673eeca9036609d13b6e193/clang/tools/clang-format/git-clang-format#L92

MSVC: https://devblogs.microsoft.com/cppblog/a-tour-of-cpp-modules-in-visual-studio/

C++20 introduces new keywords to define and consume modules and Visual Studio uses a new file type “.ixx” to define a module’s interface.

I couldn't find an authoritative source for GCC. https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Modules.html doesn't say anything.

[asottile]