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++.

Modules/CMakeCXXCompiler.cmake.in

@@ -37,7 +37,7 @@ set(CMAKE_CXX_ABI_COMPILED @CMAKE_CXX_ABI_COMPILED@)
 set(CMAKE_CXX_COMPILER_ENV_VAR "CXX")
 
 set(CMAKE_CXX_COMPILER_ID_RUN 1)
-set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;mpp;CPP;ixx;cppm)
+set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;mpp;CPP;ixx;cppm;ccm;cxxm;c++m)
 set(CMAKE_CXX_IGNORE_EXTENSIONS inl;h;hpp;HPP;H;o;O;obj;OBJ;def;DEF;rc;RC)
 
 foreach (lang C OBJC OBJCXX)

Source/cmMakefile.cxx

@@ -1229,7 +1229,7 @@ void cmMakefile::AddCustomCommandOldStyle(
 
   // Each output must get its own copy of this rule.
   cmsys::RegularExpression sourceFiles(
-    "\\.(C|M|c|c\\+\\+|cc|cpp|cxx|mpp|ixx|cppm|cu|m|mm|"
+    "\\.(C|M|c|c\\+\\+|cc|cpp|cxx|mpp|ixx|cppm|ccm|cxxm|c\\+\\+m|cu|m|mm|"
     "rc|def|r|odl|idl|hpj|bat|h|h\\+\\+|"
     "hm|hpp|hxx|in|txx|inl)$");
 

Source/cmSourceFile.h

@@ -183,8 +183,8 @@ private:
 #define CM_HEADER_REGEX "\\.(h|hh|h\\+\\+|hm|hpp|hxx|in|txx|inl)$"
 
 #define CM_SOURCE_REGEX                                                       \
-  "\\.(C|F|M|c|c\\+\\+|cc|cpp|mpp|cxx|ixx|cppm|cu|f|f90|for|fpp|ftn|m|mm|"    \
-  "rc|def|r|odl|idl|hpj|bat)$"
+  "\\.(C|F|M|c|c\\+\\+|cc|cpp|mpp|cxx|ixx|cppm|ccm|cxxm|c\\+\\+m|cu"          \
+  "|f|f90|for|fpp|ftn|m|mm|rc|def|r|odl|idl|hpj|bat)$"
 
 #define CM_PCH_REGEX "cmake_pch(_[^.]+)?\\.(h|hxx)$"
 

Source/cmake.cxx

@@ -335,7 +335,7 @@ cmake::cmake(Role role, cmState::Mode mode, cmState::ProjectKind projectKind)
     // The "c" extension MUST precede the "C" extension.
     setupExts(this->CLikeSourceFileExtensions,
               { "c", "C", "c++", "cc", "cpp", "cxx", "cu", "mpp", "m", "M",
-                "mm", "ixx", "cppm" });
+                "mm", "ixx", "cppm", "ccm", "cxxm", "c++m" });
     setupExts(this->HeaderFileExtensions,
               { "h", "hh", "h++", "hm", "hpp", "hxx", "in", "txx" });
     setupExts(this->CudaFileExtensions, { "cu" });