Swift Calling Convention: add swifterror attribute.

A ``swifterror`` attribute can be applied to a function parameter or an
AllocaInst.

This commit does not include any target-specific change. The target-specific
optimization will come as a follow-up patch.

Differential Revision: http://reviews.llvm.org/D18092

llvm-svn: 265189
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 418ab63..366b7f8 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -78,6 +78,7 @@
   isInAlloca = CS->paramHasAttr(AttrIdx, Attribute::InAlloca);
   isReturned = CS->paramHasAttr(AttrIdx, Attribute::Returned);
   isSwiftSelf = CS->paramHasAttr(AttrIdx, Attribute::SwiftSelf);
+  isSwiftError = CS->paramHasAttr(AttrIdx, Attribute::SwiftError);
   Alignment  = CS->getParamAlignment(AttrIdx);
 }