[libc++] Avoid using timespec when it might not be available
The type timespec is unconditionally used in __threading_support.
Since the C library is only required to provide it in C11, this might
cause problems for platforms with external thread porting layer (i.e.
when _LIBCPP_HAS_THREAD_API_EXTERNAL is defined) with pre-C11
In our downstream port of libc++ we used to provide a definition of
timespec in __external_threading, but this solution is not ideal
because timespec is not a reserved name.
This patch renames timespec into __libcpp_timespec_t in the
thread-related parts of libc++. For all cases except external
threading this type is an alias for ::timespec (and no functional
changes are intended).
In case of external threading it is expected that the
external_threading header will either provide a similar typedef (if
timespec is available in the vendor's C library) or provide a
definition of libcpp_timespec_t compatible with POSIX timespec.
Reviewers: ldionne, mclow.lists, EricWF
Reviewed By: ldionne
Subscribers: dexonsmith, libcxx-commits, christof, carwil
Differential Revision: https://reviews.llvm.org/D63328