Implement syncstream (p0053)

This patch implements `std::basic_syncbuf` and `std::basic_osyncstream` as specified in paper p0053r7. ~~For ease of reviewing I am submitting this patch before submitting a patch for `std::basic_osyncstream`. ~~

~~Please note, this patch is not 100% complete. I plan on adding more tests (see comments), specifically I plan on adding tests for multithreading and synchronization.~~

Edit: I decided that it would be far easier for me to keep track of this and make changes that affect both `std::basic_syncbuf` and `std::basic_osyncstream` if both were in one patch.

The patch was originally written by @zoecarver

Implements
- P0053R7 - C++ Synchronized Buffered Ostream
- LWG-3127 basic_osyncstream::rdbuf needs a const_cast
- LWG-3334 basic_osyncstream move assignment and destruction calls basic_syncbuf::emit() twice
- LWG-3570 basic_osyncstream::emit should be an unformatted output function
- LWG-3867 Should std::basic_osyncstream's move assignment operator be noexcept?

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D67086