[analyzer] Do not duplicate call graph nodes for functions that have definition and forward declaration

Patch by Ivan Sidorenko!

Differential Revision: https://reviews.llvm.org/D29643

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@295644 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit ae778dec7a29cce7246ad39a02e4e3e3ef7f6226)
2 files changed