We forward declare `reference_wrapper` in multiple places already. This
moves the declaration to the canonical place and removes unnecessary
includes of `__functional/reference_wrapper.h`.
The <__threading_support> header is a huge beast and it's really
difficult to navigate. I find myself struggling to find what I want
every time I have to open it, and I've been considering splitting it up
for years for that reason.
This patch aims not to contain any functional change. The various
implementations of the threading base are simply moved to separate
headers and then the individual headers are simplified in mechanical
ways. For example, we used to have redundant declarations of all the
functions at the top of `__threading_support`, and those are removed
since they are not needed anymore. The various #ifdefs are also
simplified and removed when they become unnecessary.
Finally, this patch adds documentation for the API we expect from any
threading implementation.
This makes <__threading_support> closer to handling only the bridge
between the system's implementation of threading and the rest of libc++.
Differential Revision: https://reviews.llvm.org/D154464
