[compiler-rt] On Windows, silence warning when building with Clang ToT

Fixes:
```
[6113/7139] Building CXX object projects\compiler-rt\lib\interception\CMakeFiles\RTInterception.x86_64.dir\interception_win.cpp.obj
C:\git\llvm-project\compiler-rt\lib\interception\interception_win.cpp(746,5): warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
  746 |     case 0xB841:  // 41 B8 XX XX XX XX : mov r8d, XX XX XX XX
      |     ^
C:\git\llvm-project\compiler-rt\lib\interception\interception_win.cpp(746,5): note: insert 'FALLTHROUGH;' to silence this warning
  746 |     case 0xB841:  // 41 B8 XX XX XX XX : mov r8d, XX XX XX XX
      |     ^
      |     FALLTHROUGH;
C:\git\llvm-project\compiler-rt\lib\interception\interception_win.cpp(746,5): note: insert 'break;' to avoid fall-through
  746 |     case 0xB841:  // 41 B8 XX XX XX XX : mov r8d, XX XX XX XX
      |     ^
      |     break;
1 warning generated.
```

Author: aganea

compiler-rt/lib/interception/interception_win.cpp

@@ -743,6 +743,7 @@ static size_t GetInstructionSize(uptr address, size_t* rel_offset = nullptr) {
     case 0x058B:  // 8B 05 XX XX XX XX : mov eax, dword ptr [XX XX XX XX]
       if (rel_offset)
         *rel_offset = 2;
+      FALLTHROUGH;
     case 0xB841:  // 41 B8 XX XX XX XX : mov r8d, XX XX XX XX
       return 6;