Add Host::MAX_THREAD_NAME_LENGTH constant.

This value gets set to a max uint32_t value when there is no known limit; otherwise,
it is set to a value appropriate for the platform.  For the moment, only
Linux, FreeBSD and NetBSD set it to 16.  All other platforms set it to
the max uint32_t value.

Modifies the Process private state thread names to fit within a 16-character limit
when the max thread name length is <= 16.  These guarantee that the thread names
can be distinguished within the first 16 characters.  Prior to this change, those
threads had names in the final dotted name segment that were not distinguishable
within the first 16 characters.

llvm-svn: 213183