[bolt] Fix MSVC builds

[smeenai]

Summary

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

[maksfb]

Thanks for the fix and the detailed info.

[ayermolo]

thanks for the fix

bolt/include/bolt/Core/DIEBuilder.h

struct DWARFUnitInfo {
     DIE *UnitDie = nullptr;
     uint32_t UnitId = 0;
     uint32_t UnitOffset = 0;
     uint32_t UnitLength = 0;
     bool IsConstructed = false;
     // A map of DIE offsets in original DWARF section to DIE ID.
     // Whih is used to access DieInfoVector.
     std::unordered_map<uint64_t, uint32_t> DIEIDMap;
+
+    // Some STL implementations don't have a noexcept move constructor for
+    // unordered_map (e.g. https://github.com/microsoft/STL/issues/165 explains
+    // why the Microsoft STL doesn't). In that case, the default move
+    // constructor generated for DWARFUnitInfo isn't noexcept either, and thus
+    // resizing a vector of DWARFUnitInfo will copy elements instead of moving
+    // them (https://en.cppreference.com/w/cpp/utility/move_if_noexcept).
+    // DWARFUnitInfo isn't copyable though, since the DieInfoVector member is a
+    // vector of unique_ptrs and unique_ptr isn't copyable, so using a vector of
+    // DWARFUnitInfo causes build errors. Explicitly marking DWARFUnitInfo as
+    // non-copyable forces vector resizes to move instead and fixes the issue.
+    DWARFUnitInfo() = default;
+    DWARFUnitInfo(const DWARFUnitInfo &) = delete;
+    DWARFUnitInfo(DWARFUnitInfo &&) = default;
   };
 
   enum class ProcessingType { DWARF4TUs, DWARF5TUs, CUs };
 
 private:
   /// Contains information so that we we can update references in locexpr after
   /// we calculated all the final DIE offsets.
   struct LocWithReference {