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/Core/HW/AddressSpace.cpp

@@ -303,7 +303,7 @@ private:
 
 struct SmallBlockAccessors : Accessors
 {
-  SmallBlockAccessors(u8** alloc_base, u32 size) : alloc_base(alloc_base), size(size) {}
+  SmallBlockAccessors(u8** alloc_base_, u32 size_) : alloc_base{alloc_base_}, size{size_} {}
 
   bool IsValidAddress(u32 address) const override
   {