Make wpi::condition_variable typedef to std::condition_variable_any if
wpi::mutex typedefs to priority_mutex.
priority_condition_variable was originally intended as a copy of
std::condition_variable_any that also returned the internal handle like
std::condition_variable. This was needed because NetComm required a
pthread_cond_t. We no longer use it anywhere.
Its args were specialized for priority_mutex, but
std::condition_variable_any supports this and more through
templatization.
During shared library loading, a different libLLVM can be pulled in, causing
llvm symbols from dependent libraries to resolve to that library instead of
this one. This has been seen in the wild with the Mesa OpenGL implementation
in JavaFX applications (see wpilibsuite/shuffleboard#361).
This is clearly a very breaking change. For some level of backwards
compatibility, a namespace alias from llvm to wpi is performed in the "llvm"
headers. Unfortunately, forward declarations of llvm classes will still break,
but compilers seem to generate clear error messages in those cases
("namespace alias 'llvm' not allowed here, assuming 'wpi'").
This change also moves all the wpiutil headers to a single "wpi" subdirectory
from the previously split "llvm", "support", "tcpsockets", and "udpsockets".
Shim headers will be added for backwards compatibility in a later commit.
