Revert r347823 "[TextAPI] Switch back to a custom Platform enum."

It broke the Windows buildbots, e.g.
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/21829/steps/test/logs/stdio

This also reverts the follow-ups: r347824, r347827, and r347836.

llvm-svn: 347874