debuginfo: Added description of algorithm for handling recursive types.

Also fixed nasty bug caused by calling LLVMDIBuilderCreateStructType() with a null pointer where an empty array was expected (which would trigger an unintelligable assertion somewhere down the line).

src/rustllvm/RustWrapper.cpp

@@ -790,29 +790,6 @@ extern "C" LLVMValueRef LLVMDIBuilderCreateNameSpace(
         LineNo));
 }
 
-// extern "C" LLVMValueRef LLVMDIBuilderCreateForwardDecl(
-//     DIBuilderRef Builder,
-//     unsigned Tag,
-//     const char* Name,
-//     LLVMValueRef Scope,
-//     LLVMValueRef File,
-//     unsigned Line,
-//     unsigned RuntimeLang,
-//     uint64_t SizeInBits,
-//     uint64_t AlignInBits)
-// {
-//     return wrap(Builder->createForwardDecl(
-//         Tag,
-//         Name,
-//         unwrapDI<DIDescriptor>(Scope),
-//         unwrapDI<DIFile>(File),
-//         Line,
-//         RuntimeLang,
-//         SizeInBits,
-//         AlignInBits
-//     ));
-// }
-
 extern "C" void LLVMDICompositeTypeSetTypeArray(
     LLVMValueRef CompositeType,
     LLVMValueRef TypeArray)