Re-add <experimental/coroutine> to the module map.

The original issues were caused because <experimental/coroutine>
didn't correctly #ifdef out enough of the header, which caused incomplete
types to be used.

This patch fixes the `#if defined(__cpp_coroutines)` guard and re-adds
the headers to the module map.

It also uglifies some incorrectly non-reserved names.

llvm-svn: 303936