Xcode: Rename internal variable {Platform => Generator}Toolset

The latter matches with CMAKE_GENERATOR_TOOLSET better.
VS: Split user- and generator-provided PlatformToolset

Divide the cmGlobalVisualStudio10Generator "PlatformToolset" member into
two members representing the generator-selected default toolset and the
user-specified CMAKE_GENERATOR_TOOLSET value.  Prefer the user-specified
value, if any, and then fall back to the generator-selected default.
Allow a toolchain file to specify a generator toolset

Delay use of CMAKE_GENERATOR_TOOLSET until the CMakeSystem.cmake
file has been configured and loaded during the first project() or
enable_language() command.  This gives the toolchain file named by
CMAKE_TOOLCHAIN_FILE a chance to set CMAKE_GENERATOR_TOOLSET.  This
point is still early enough to set the generator toolset prior to
the initialization of any languages that might use the toolset.

The cmake::GeneratorToolset member variable remains an indication
of what was specified by the -T option or loaded from the cache.
It does not need to be updated based on the toolchain file setting.
The cmMakefile::TryCompile can still pass cmake::GeneratorToolset
into the inner instance because the try-compiled project will do
platform and language initialization using the CMakeSystem module
configured for the outer project.

Extend the RunCMake.GeneratorToolset test with cases that use a
toolchain file to set CMAKE_GENERATOR_TOOLSET.
Merge topic 'delay-generator-toolset'

528e8af1 Allow a toolchain file to specify a generator toolset
98afb454 VS: Split user- and generator-provided PlatformToolset
3e9f6e36 Xcode: Rename internal variable {Platform => Generator}Toolset