Clang: Add separate "AppleClang" compiler id
Apple distributes their own Clang build with their own version numbers that differ from upstream Clang. Use the __apple_build_version__ symbol to identify the Apple Clang compiler and report the Apple Build Version as the fourth version component in CMAKE_<LANG>_COMPILER_VERSION. Add Compiler/AppleClang-<lang> and Platform/Darwin-AppleClang-<lang> modules that simply include the upstream equivalents. Fix comparisons of CMAKE_<LANG>_COMPILER_ID to Clang in CMake's own source and tests to account for AppleClang.