[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
diff --git a/lib/StaticAnalyzer/Checkers/StreamChecker.cpp b/lib/StaticAnalyzer/Checkers/StreamChecker.cpp
index 94ff266..2f96bbf 100644
--- a/lib/StaticAnalyzer/Checkers/StreamChecker.cpp
+++ b/lib/StaticAnalyzer/Checkers/StreamChecker.cpp
@@ -115,10 +115,7 @@
 }
 
 bool StreamChecker::evalCall(const CallExpr *CE, CheckerContext &C) const {
-  const ProgramState *state = C.getState();
-  const Expr *Callee = CE->getCallee();
-  SVal L = state->getSVal(Callee);
-  const FunctionDecl *FD = L.getAsFunctionDecl();
+  const FunctionDecl *FD = C.getCalleeDecl(CE);
   if (!FD)
     return false;