Add front-end infrastructure now address space casts are in LLVM IR.

With the introduction of explicit address space casts into LLVM, there's
a need to provide a new cast kind the front-end can create for C/OpenCL/CUDA
and code to produce address space casts from those kinds when appropriate.

Patch by Michele Scandale!

llvm-svn: 197036
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 390cfe9..60a39a5 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -7120,6 +7120,7 @@
   case CK_BuiltinFnToFnPtr:
   case CK_ZeroToOCLEvent:
   case CK_NonAtomicToAtomic:
+  case CK_AddressSpaceConversion:
     llvm_unreachable("invalid cast kind for integral value");
 
   case CK_BitCast:
@@ -7592,6 +7593,7 @@
   case CK_BuiltinFnToFnPtr:
   case CK_ZeroToOCLEvent:
   case CK_NonAtomicToAtomic:
+  case CK_AddressSpaceConversion:
     llvm_unreachable("invalid cast kind for complex value");
 
   case CK_LValueToRValue: