[bolt] Fix MSVC builds

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

Reviewed By: ayermolo, maksfb

Differential Revision: https://reviews.llvm.org/D154924