cmake: Split -E command implementation into separate source file

Move the cmake::ExecuteCMakeCommand static method and all the static
methods it calls out of the 'cmake' class to a separate 'cmcmd' class.
Build the latter as part of the main cmake executable with cmakemain.cxx
and not in CMakeLib.  Drop unused header includes from "cmake.cxx".

By moving this implementation out of cmake.cxx we avoid carrying it
around in all the executables that use class 'cmake'.  It is needed only
for the main "cmake -E" functionality.
CMake Nightly Date Stamp
Merge topic 'split-cmake-E' into next

c04995b cmake: Split -E command implementation into separate source file
CMakeDetermineCompilerId: Fix local var init

Add initialization of the ARCHITECTURE_ID local and fix the
initialization of the COMPILER_ID_TWICE local.
CMakeDetermineCompilerId: Add notion of "simulated" id/version

Some compilers try to simulate other compilers as a drop-in replacement
supporting all the same command-line options and predefined preprocessor
macros.  In such cases it will be useful to have CMake load the compiler
information files for the simulated compiler instead of duplicating the
information.  Teach CMakeDetermineCompilerId to extract the simulated
compiler id and version when the compiler id detection provides it.
Clang: Support Windows variants for GNU and MSVC (#13035, #14458)

Teach the compiler identification preprocessor tests to report when
Clang simulates MSVC, and what version.  If not MSVC, assume GNU.

Teach compiler information modules Clang-(C|CXX) to recognize when Clang
simulates MSVC and skip loading the GNU information.

Teach the Windows-MSVC platform information to recognize when it is
loaded as the simulated compiler and use that version information
instead of the real compiler's (different) version scheme.

Add platform modules Windows-Clang-(C|CXX) and support module
Windows-Clang to load either Windows-MSVC or Windows-GNU and wrap
the corresponding information macros.
Merge topic 'clang-cl-compiler' into next

3d8356d Clang: Support Windows variants for GNU and MSVC (#13035, #14458)
51ab85c CMakeDetermineCompilerId: Add notion of "simulated" id/version
be10826 CMakeDetermineCompilerId: Fix local var init
e20edcd CMake Nightly Date Stamp