Genex: Enable COMPILE_LANGUAGE for COMPILE_OPTIONS with Visual Studio

Since commit v3.9.0-rc4~3^2~1 (VS: Fix target_compile_options for CUDA,
2017-06-21), the evaluation of `COMPILE_LANGUAGE` receives the proper
language.  The set of compile flags used for a target's C and C++
sources is based on the linker language.  By default this is always the
C++ flags if any C++ sources appear in the target, and otherwise the C
flags.  Therefore we can define the `COMPILE_LANGUAGE` generator
expression in `COMPILE_OPTIONS` to match the selected language.

This is not exactly the same as for other generators, but is the best VS
can do.  It is also sufficient for many use cases since the set of
allowed flags for C and C++ is almost the same in Visual Studio.
Furthermore, since the VS generator moves many of the flags to
declarative `.vcxproj` elements, it will automatically avoid passing
C++ flags for C sources.

Issue: #17435

Help/manual/cmake-generator-expressions.7.rst

@@ -97,7 +97,7 @@ Available logical expressions are:
   compile features and a list of supported compilers.
 ``$<COMPILE_LANGUAGE:lang>``
   ``1`` when the language used for compilation unit matches ``lang``,
-  otherwise ``0``.  This expression used to specify compile options for
+  otherwise ``0``.  This expression may be used to specify compile options for
   source files of a particular language in a target. For example, to specify
   the use of the ``-fno-exceptions`` compile option (compiler id checks
   elided):
@@ -109,10 +109,12 @@ Available logical expressions are:
       PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
     )
 
-  This generator expression has limited use because it is not possible to
-  use it with the Visual Studio generators.  Portable buildsystems would
-  not use this expression, and would create separate libraries for each
-  source file language instead:
+  Note that with :ref:`Visual Studio Generators` there is no way to represent
+  target-wide flags separately for ``C`` and ``CXX`` languages.  Under these
+  generators, target-wide flags for both C and C++ sources will be evaluated
+  using ``CXX`` if there are any C++ sources and otherwise using ``C``.
+  A workaround is to create separate libraries for each source file language
+  instead:
 
   .. code-block:: cmake
 

Help/release/dev/extend-compile-language-genex.rst

@@ -0,0 +1,7 @@
+extend-compile-language-genex
+-----------------------------
+
+* The ``COMPILE_LANGUAGE`` :manual:`generator expression
+  <cmake-generator-expressions(7)>` may now be used with
+  :ref:`Visual Studio Generators` in :prop_tgt:`COMPILE_OPTIONS`
+  and :command:`file(GENERATE)`.