[compiler-rt][asan] Silence MSVC warning

```
[3/56] Building CXX object projects\compiler-rt\lib\asan\CMakeFiles\RTAsan.x86_64.dir\asan_poisoning.cpp.obj
C:\git\llvm-project\compiler-rt\lib\asan\asan_poisoning.cpp(450): warning C4390: ';': empty controlled statement found; is this the intent?
[4/56] Building CXX object projects\compiler-rt\lib\asan\CMakeFiles\RTAsan_dynamic.x86_64.dir\asan_poisoning.cpp.obj
C:\git\llvm-project\compiler-rt\lib\asan\asan_poisoning.cpp(450): warning C4390: ';': empty controlled statement found; is this the intent?
```