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).

Source/cmGeneratorTarget.h

@@ -1196,4 +1196,34 @@ public:
     bool operator()(cmGeneratorTarget const* t1,
                     cmGeneratorTarget const* t2) const;
   };
+
+  // C++20 module support queries.
+
+  /**
+   * Query whether the target expects C++20 module support.
+   *
+   * This will inspect the target itself to see if C++20 module
+   * support is expected to work based on its sources.
+   */
+  bool HaveCxx20ModuleSources() const;
+
+  enum class Cxx20SupportLevel
+  {
+    // C++ is not available.
+    MissingCxx,
+    // The experimental feature is not available.
+    MissingExperimentalFlag,
+    // The target does not require at least C++20.
+    NoCxx20,
+    // C++20 modules are available and working.
+    Supported,
+  };
+  /**
+   * Query whether the target has C++20 module support available (regardless of
+   * whether it is required or not).
+   */
+  Cxx20SupportLevel HaveCxxModuleSupport(std::string const& config) const;
+
+  // Check C++ module status for the target.
+  void CheckCxxModuleStatus(std::string const& config) const;
 };