Restructured code, no functional changes.

Summary:
Moved implementation of classes declared in
ClangTidyDiagnosticConsumer.h to ClangTidyDiagnosticConsumer.cpp.
Added a FIXME note in ClangTidyDiagnosticConsumer::HandleDiagnostic.

Reviewers: klimek, djasper

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2520