Help: Add documentation for CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT

Signed-off-by: Deniz Bahadir <deniz@code.bahadir.email>

Help/manual/cmake-variables.7.rst

@@ -71,6 +71,7 @@ Variables that Provide Information
    /variable/CMAKE_JOB_POOL_PRECOMPILE_HEADER
    /variable/CMAKE_JOB_POOLS
    /variable/CMAKE_LANG_COMPILER_AR
+   /variable/CMAKE_LANG_COMPILER_FRONTEND_VARIANT
    /variable/CMAKE_LANG_COMPILER_RANLIB
    /variable/CMAKE_LANG_LINK_LIBRARY_SUFFIX
    /variable/CMAKE_LINK_LIBRARY_SUFFIX

Help/variable/CMAKE_LANG_COMPILER_FRONTEND_VARIANT.rst

+CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT
+--------------------------------------
+
+.. versionadded:: 3.14
+
+Identification string of the compiler frontend variant.
+
+Some compilers have multiple, different frontends for accepting command
+line options.  (For example ``Clang`` originally only had a frontend
+compatible with the ``GNU`` compiler but since its port to Windows
+(``Clang-Cl``) it now also supports a frontend compatible with ``MSVC``.)
+When CMake detects such a compiler it sets this
+variable to what would have been the :variable:`CMAKE_<LANG>_COMPILER_ID` for
+the compiler whose frontend it resembles.
+
+.. note::
+  In other words, this variable describes what command line options
+  and language extensions the compiler frontend expects.