PR19623: Implement typedefs of void.

This the LLVM portion that will allow Clang and other frontends to emit
typedefs of void by providing a null type for the typedef's underlying
type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207777 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/DIBuilder.cpp

@@ -355,7 +355,6 @@ DIDerivedType DIBuilder::createReferenceType(unsigned Tag, DIType RTy) {
 DIDerivedType DIBuilder::createTypedef(DIType Ty, StringRef Name, DIFile File,
                                        unsigned LineNo, DIDescriptor Context) {
   // typedefs are encoded in DIDerivedType format.
-  assert(Ty.isType() && "Invalid typedef type!");
   Value *Elts[] = {
     GetTagConstant(VMContext, dwarf::DW_TAG_typedef),
     File.getFileNode(),