[analyzer] Refactor checkers to use helper function for getting callee Decl and name.

We are getting name of the called function or it's declaration in a few checkers. Refactor them to use the helper function in the CheckerContext. 

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145576 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed