Change LocationContextMap to be a temporary instead of shared variable in BugReporter.

BugReporter is used to process ALL bug reports.  By using a shared map,
we are having mappings from different PathDiagnosticPieces to LocationContexts
well beyond the point where we are processing a given report.  This
state is inherently error prone, and is analogous to using a global
variable.  Instead, just create a temporary map, one per report,
and when we are done with it we throw it away.  No extra state.