swift234

[cmake] Refactor out cmark related config from swift_common_standalone_build_config to swift_common_standalone_build_config_cmark.

This is apart of a series of patches that separates configuration of
llvm/clang/cmark in swift_common_standalone_build_config. Right now each project
has configuration occurring in a mixed up way. By refactoring it becomes much
easier to read the code.

rdar://26154980