c++: add module extensions

There is a trend in the broader C++ community to use a different extension
for module interface units, even though (in GCC) they are compiled in the
same way as other source files.  Let's recognize these extensions as C++.

.ixx is the MSVC standard, while the .c*m are supported by Clang.  libc++
standard headers use .cppm, as their other source files use .cpp.
Perhaps libstdc++ might use .ccm for parallel consistency?

One issue with .c++m is that libcpp/mkdeps.cc has been using it for the
phony dependencies to express module dependencies, so I'm changing mkdeps to
something less likely to be an actual file, ".c++-module".

gcc/cp/ChangeLog:

	* lang-specs.h: Add module interface extensions.

gcc/ChangeLog:

	* doc/invoke.texi: Update module extension docs.

libcpp/ChangeLog:

	* mkdeps.cc (make_write): Change .c++m to .c++-module.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dep-1_a.C
	* g++.dg/modules/dep-1_b.C
	* g++.dg/modules/dep-2.C: Change .c++m to .c++-module.

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/cp/lang-specs.h b/gcc/cp/lang-specs.h
index 7a7f5ff..e565156 100644
--- a/gcc/cp/lang-specs.h
+++ b/gcc/cp/lang-specs.h
@@ -39,6 +39,12 @@ along with GCC; see the file COPYING3.  If not see
   {".HPP", "@c++-header", 0, 0, 0},
   {".tcc", "@c++-header", 0, 0, 0},
   {".hh",  "@c++-header", 0, 0, 0},
+  /* Module interface unit.  Should there also be a .C counterpart?  */
+  {".ixx", "@c++", 0, 0, 0}, /* MSVC */
+  {".cppm", "@c++", 0, 0, 0}, /* Clang/libc++ */
+  {".cxxm", "@c++", 0, 0, 0},
+  {".c++m", "@c++", 0, 0, 0},
+  {".ccm", "@c++", 0, 0, 0},
   {"@c++-header",
       "%{E|M|MM:cc1plus -E %{fmodules-ts:-fdirectives-only -fmodule-header}"
       "  %(cpp_options) %2 %(cpp_debug_options)}"