DRTVWR-447: Attempt to post BugSplat metadata with Mac crash reports.

Introduce CrashMetadata, an LLSingleton in llappviewermacosx.cpp, declared in
llappviewermacosx-for-objc.h and accessed by the various
BugsplatStartupManagerDelegate override methods. CrashMetadata is populated by
reading the previous (presumably crashed) run's static_debug_info.log file.

This replaces the previous getOldLogFilePathname(), getFatalMessage() and
getAgentFullname() functions. To extend that suite for additional metadata,
not only would we have to keep adding new free functions, but we'd have to
keep rereading the static_debug_info.log file.

Override the new applicationKeyForBugsplatStartupManager,
defaultUserNameForBugsplatStartupManager,
defaultUserEmailForBugsplatStartupManager methods to extract relevant fields
from CrashMetadata. Change applicationLogForBugsplatStartupManager and
attachmentForBugsplatStartupManager to do the same.

Enhance llviewerregion.cpp to update the static_debug_info.log file every
time we enter a new region.