Unbreak -Werror=implicit-fallthrough with Clang

In file included from src/input_common/sdl/sdl_impl.cpp:16:
In file included from /usr/local/include/SDL2/SDL.h:32:
In file included from /usr/local/include/SDL2/SDL_main.h:25:
/usr/local/include/SDL2/SDL_stdinc.h:445:9: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case 3:         *_p++ = _val;   /* fallthrough */
        ^
/usr/local/include/SDL2/SDL_stdinc.h:445:9: note: insert '[[fallthrough]];' to silence this warning
        case 3:         *_p++ = _val;   /* fallthrough */
        ^
        [[fallthrough]];
/usr/local/include/SDL2/SDL_stdinc.h:445:9: note: insert 'break;' to avoid fall-through
        case 3:         *_p++ = _val;   /* fallthrough */
        ^
        break;
/usr/local/include/SDL2/SDL_stdinc.h:446:9: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case 2:         *_p++ = _val;   /* fallthrough */
        ^
/usr/local/include/SDL2/SDL_stdinc.h:446:9: note: insert '[[fallthrough]];' to silence this warning
        case 2:         *_p++ = _val;   /* fallthrough */
        ^
        [[fallthrough]];
/usr/local/include/SDL2/SDL_stdinc.h:446:9: note: insert 'break;' to avoid fall-through
        case 2:         *_p++ = _val;   /* fallthrough */
        ^
        break;
/usr/local/include/SDL2/SDL_stdinc.h:447:9: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case 1:         *_p++ = _val;   /* fallthrough */
        ^
/usr/local/include/SDL2/SDL_stdinc.h:447:9: note: insert '[[fallthrough]];' to silence this warning
        case 1:         *_p++ = _val;   /* fallthrough */
        ^
        [[fallthrough]];
/usr/local/include/SDL2/SDL_stdinc.h:447:9: note: insert 'break;' to avoid fall-through
        case 1:         *_p++ = _val;   /* fallthrough */
        ^
        break;
3 errors generated.

video_core: gl_shader_decompiler: Fix implicit fallthrough errors.

loader/nso: Resolve moves not occurring in DecompressSegment

service: hid: npad: Fix implicit fallthrough errors.

service/time: Minor changes

memory/slab_heap: Make use of static_cast over reinterpret_cast

This is already initialized within the class body.

This doesn't modify internal member state.

Many of these implementations are used to implement a polymorphic
interface. While not directly used polymorphically, this prevents
virtual destruction from ever becoming an issue.

externals: Update fmt to 6.2.0

common/file_util: Allow access to files on network shares

Casting from void* with static_cast is permitted by the standard, so we
can just make use of that instead.

Given the std::vector was const, an automatic move out of the function
could not occur.

We can allow automatic return value optimizations to occur by making the
buffer non-const.

video_code: Fix implicit switch fallthrough.

Since yesterday, this breaks the build on linux.
So let's fix it.

Keeps the library up to date.

Revert "gl_shader_cache: Use CompileDepth::FullDecompile on GLSL"

file_util: Early-exit in WriteArray and ReadArray if specified lengths are zero

Implement a new virtual memory manager

gl_query_cache: Resolve use-after-move in CachedQuery move assignment operator

- Should be no functional changes.

- Includes removing some service impls. that are untested.

- Includes removing some SVC impls. that are untested.

- Fixes Super Smash Bros. Ultimate.