Replace ProgramPoint llvm::cast support to be well-defined.

See r175462 for another example/more details.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175812 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/StaticAnalyzer/Checkers/CStringChecker.cpp b/lib/StaticAnalyzer/Checkers/CStringChecker.cpp
index d0c4322..cc55e9f 100644
--- a/lib/StaticAnalyzer/Checkers/CStringChecker.cpp
+++ b/lib/StaticAnalyzer/Checkers/CStringChecker.cpp
@@ -281,7 +281,7 @@
   DefinedOrUnknownSVal Size = Extent.castAs<DefinedOrUnknownSVal>();
 
   // Get the index of the accessed element.
-  DefinedOrUnknownSVal Idx = cast<DefinedOrUnknownSVal>(ER->getIndex());
+  DefinedOrUnknownSVal Idx = ER->getIndex().castAs<DefinedOrUnknownSVal>();
 
   ProgramStateRef StInBound = state->assumeInBound(Idx, Size, true);
   ProgramStateRef StOutBound = state->assumeInBound(Idx, Size, false);