Refactoring of constant expression evaluator

This introduces a generic base class for the expression evaluator
classes, which handles a few common expression types which were
previously handled separately in each class.  Also, the expression
evaluator now uses ConstStmtVisitor.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131281 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/CFG.cpp b/lib/Analysis/CFG.cpp
index 77c30dd..f120191 100644
--- a/lib/Analysis/CFG.cpp
+++ b/lib/Analysis/CFG.cpp
@@ -443,7 +443,7 @@
       return Result.Val.getInt().getBoolValue();
 
     if (Result.Val.isLValue()) {
-      Expr *e = Result.Val.getLValueBase();
+      const Expr *e = Result.Val.getLValueBase();
       const CharUnits &c = Result.Val.getLValueOffset();        
       if (!e && c.isZero())
         return false;