AnalysisConsumer: use canonical decl for both lookup and store of

visited decls.

Due to redeclarations, the function may have different declarations used
in CallExpr and in the definition. However, we need to use a unique
declaration for both store and lookup in VisitedCallees. This patch
fixes issues with analysis in topological order. A simple test is
included.

Patch by Alex Sidorin!

Differential Revision: http://reviews.llvm.org/D15410

llvm-svn: 257318