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, 17 insertions, 0 deletions

diff --git a/Tests/RunCMake/CXXModules/NotCXXSourceModules-stderr.txt b/Tests/RunCMake/CXXModules/NotCXXSourceModules-stderr.txt
new file mode 100644
index 0000000..d341c1f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCXXSourceModules-stderr.txt
@@ -0,0 +1,17 @@
+CMake Warning \(dev\) at NotCXXSourceModules.cmake:7 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:6 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Error in CMakeLists.txt:
+  Target "not-cxx-source" contains the source
+
+    .*/Tests/RunCMake/CXXModules/sources/c-anchor.c
+
+  in a file set of type "CXX_MODULES" but the source is not classified as a
+  "CXX" source.
+
+
+CMake Generate step failed.  Build files cannot be regenerated correctly.