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/ArrayBoundChecker.cpp b/lib/StaticAnalyzer/Checkers/ArrayBoundChecker.cpp
index 051d60a..312bc74 100644
--- a/lib/StaticAnalyzer/Checkers/ArrayBoundChecker.cpp
+++ b/lib/StaticAnalyzer/Checkers/ArrayBoundChecker.cpp
@@ -44,7 +44,7 @@
     return;
 
   // Get the index of the accessed element.
-  DefinedOrUnknownSVal Idx = cast<DefinedOrUnknownSVal>(ER->getIndex());
+  DefinedOrUnknownSVal Idx = ER->getIndex().castAs<DefinedOrUnknownSVal>();
 
   // Zero index is always in bound, this also passes ElementRegions created for
   // pointer casts.