[CallGraph] Make sure the edges are not missed due to re-declarations

A patch by Daniel DeFreez!

We were previously dropping edges on re-declarations. Store the
canonical declarations in the graph to ensure that different
references to the same function end up reflected with the same call graph
node.

(Note, this might lead to performance fluctuation because call graph
is used to determine the function analysis order.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224398 91177308-0d34-0410-b5e6-96231b3b80d8