Make clang::ento::CreateAnalysisConsumer a part of a public interface of the static analyzer.

Summary:
This allows for a better alternative to the FrontendAction hack used in
clang-tidy in order to get static analyzer's ASTConsumer.

Reviewers: jordan_rose, krememek

Reviewed By: jordan_rose

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2505