[CSA] [NFC] Move AnalysisContext.h to AnalysisDeclContext.h

The implementation is in AnalysisDeclContext.cpp and the class is called
AnalysisDeclContext.

Making those match up has numerous benefits, including:

 - Easier jump from header to/from implementation.
 - Easily identify filename from class.

Differential Revision: https://reviews.llvm.org/D37500

llvm-svn: 312671