We need to explicitly mark DWARFUnitInfo as non-copyable since MSVC's
STL has a noexcept(false) move constructor for unordered_map; see
the added comment for more details.
An alternative might be using SmallVector instead of std::vector, since
that never tries to copy elements [1]. That would result in a bunch of
API changes though, so I figured a smaller targeted fix was better.
[1] https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h