Fix several warnings

A small, nonexhaustive set of warning fixes. The DiscIO Volume change
is a workaround for a GCC bug [1] that causes returning an unengaged
std::optional to emit annoying -Wmaybe-uninitialized warnings.
This last change alone fixes pages upon pages of warnings since
Volume.h is included from several files.

-Wstringop-truncation is another irrelevant warning for us, but
unfortunately there seems to be no way to disable it without
adding ugly pragmas wherever the warning appears.

Source/Core/DiscIO/Volume.h

@@ -50,7 +50,7 @@ public:
   {
     T temp;
     if (!Read(offset, sizeof(T), reinterpret_cast<u8*>(&temp), partition))
-      return {};
+      return std::nullopt;
     return Common::FromBigEndian(temp);
   }
   std::optional<u64> ReadSwappedAndShifted(u64 offset, const Partition& partition) const