core: Make variable shadowing a compile-time error

Now that we have most of core free of shadowing, we can enable the
warning as an error to catch anything that may be remaining and also
eliminate this class of logic bug entirely.

src/core/file_sys/vfs_layered.h

@@ -13,7 +13,7 @@ namespace FileSys {
 // one and falling back to the one after. The highest priority directory (overwrites all others)
 // should be element 0 in the dirs vector.
 class LayeredVfsDirectory : public VfsDirectory {
-    LayeredVfsDirectory(std::vector<VirtualDir> dirs, std::string name);
+    explicit LayeredVfsDirectory(std::vector<VirtualDir> dirs_, std::string name_);
 
 public:
     ~LayeredVfsDirectory() override;