Works towards P0619R4.

- `std::uncaught_exception` was not previously deprecated. This patch deprecates it since C++17 as per N4259. `std::uncaught_exceptions` is used instead as libc++ unconditionally provides this function.
- `std::get_temporary_buffer` is replaced with the internal version `__get_temporary_buffer`.
- `std::return_temporary_buffer` is replaced with direct `__libcpp_deallocate_unsized` call.

Escape hatches:
- `_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION` restores `std::uncaught_exception`.
- `_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER` restores `std::get_temporary_buffer` and `std::return_temporary_buffer`.

# Conflicts:
#	libcxx/docs/Status/Cxx17Papers.csv

# Conflicts:
#	libcxx/docs/ReleaseNotes/20.rst
#	libcxx/docs/Status/Cxx20.rst

# Conflicts:
#	libcxx/docs/Status/Cxx20.rst

- C++03 `> >`
- Module exporting
- `unique_ptr` construction