DRTVWR-447: Suppress BugSplat UI; auto-fill certain BugSplat data.

Direct BugSplat to send crash reports without prompting, on both Windows and
Mac.

Add a mechanism by which code called after LL_ERRS() can retrieve the fatal
log message string. (How did the crash logger extract that for Linden crash
logging?)

Add that fatal message to crash reports on Windows. But as BugsplatMac is
engaged only on the run _after_ the crash, we no longer have that message in
memory.

Also add user name and region location to Windows crash reports. On Mac, (a)
we don't have the information from the previous run and (b) BugsplatMac
doesn't provide an API to attach that information to the crash report.

Add Mac logging to indicate the success or failure of sending the crash
report. Add Windows logging to indicate we're about to send.