Fix a compiler error under MSVC

Error message was:
CGDebugInfo.cpp(1047) : error C2666: 'llvm::MDTypeRefArray::operator []' : 2 overloads have similar conversions
  DebugInfoMetadata.h(106): could be 'llvm::MDTypeRef llvm::MDTypeRefArray::operator [](unsigned int) const'
  while trying to match the argument list '(llvm::DITypeArray, int)'

llvm-svn: 234308