Create ConstantExpr class

A ConstantExpr class represents a full expression that's in a context where a
constant expression is required. This class reflects the path the evaluator
took to reach the expression rather than the syntactic context in which the
expression occurs.

In the future, the class will be expanded to cache the result of the evaluated
expression so that it's not needlessly re-evaluated

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D53475

llvm-svn: 345692
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index d276ccc..99a9e87 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -3507,6 +3507,10 @@
   case Expr::CXXInheritedCtorInitExprClass:
     llvm_unreachable("unexpected statement kind");
 
+  case Expr::ConstantExprClass:
+    E = cast<ConstantExpr>(E)->getSubExpr();
+    goto recurse;
+
   // FIXME: invent manglings for all these.
   case Expr::BlockExprClass:
   case Expr::ChooseExprClass: