Allow specifying a custom PathDiagnosticConsumer for use with the static analyzer.

Summary:
Make objects returned by CreateAnalysisConsumer expose an interface,
that allows providing a custom PathDiagnosticConsumer, so that users can have
raw data in a form easily usable from the code (unlike plist/HTML in a file).

Reviewers: jordan_rose, krememek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2556