We should use `orc_rt` as the public C++ API namespace for the ORC runtime and
control symbol visibility to hide implementation details, rather than rely on
the '__' prefix.
Inspired by LLVM_DEBUG, but using environment variables rather than command line
options.
Code can use ORC_RT_DEBUG(...) (if ORC_RT_DEBUG_TYPE is set), or
ORC_RT_DEBUG_WITH_TYPE(<type>, ...) (if ORC_RT_DEBUG_TYPE is not set. E.g. in
headers).
Debug logging is enabled in the executor by setting the ORC_RT_DEBUG environment
variable. Debug logging can be restricted by type by setting the
ORC_RT_DEBUG_TYPES environment variable to a comma separated list of types,
e.g. ORC_RT_DEBUG_TYPES=macho_platform,sps.
Differential Revision: https://reviews.llvm.org/D116139
