Add a new LC_NOTE for Mach-O corefiles, "proces metadata", which is a
JSON string. Currently there may be a `threads` key in the JSON,
and if `threads` is present, it is an array with the same number of
elements as there are LC_THREADs in the corefile. This patch adds
support for a `thread_id` key-value for each `thread` entry, to
supply a thread ID for that LC_THREAD.
Differential Revision: https://reviews.llvm.org/D158785
rdar://113037252
2cab996192