Rework how PathDiagnosticConsumers pass knowledge of what files they

generated for a given diagnostic to another.  Because PathDiagnostics
are specific to a give PathDiagnosticConsumer, store in
a FoldingSet a unique hash for a PathDiagnostic (that will be the same
for the same bug for different PathDiagnosticConsumers) that
stores a list of files generated.  This can then be read by the
other PathDiagnosticConsumers.

This fixes breakage in the PLIST-HTML output.

llvm-svn: 162580