Added virtual method DiagnosticClient::IncludeInDiagnosticCounts().  This is used by Diagnostics to determine if a diagnostic sent to a given DiagnosticClient should be included in the count of diagnostics.  The default implementation of this method returns 'true'.

Implemented DiagCollector::IncludeInDiagnosticCounts() to return 'false' so that the batching of diagnostics for use with BugReporter doesn't mess up the count of real diagnostics.

llvm-svn: 62873