cmTarget: add support for C++ module fileset types

C++ modules have two variants which are of importance to CMake:

  - `CXX_MODULES`: interface modules (those using `export module M;`,
    `export module M:part;`, or `module M:internal_part;`)
  - `CXX_MODULE_HEADER_UNITS`: importable header units

Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).

1 files changed, 4 insertions, 0 deletions

diff --git a/Tests/RunCMake/CXXModules/sources/cxx-anchor.cxx b/Tests/RunCMake/CXXModules/sources/cxx-anchor.cxx
new file mode 100644
index 0000000..9c94ec1
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/cxx-anchor.cxx
@@ -0,0 +1,4 @@
+int cxx_anchor()
+{
+  return 0;
+}