[Support] Silence warning with Clang ToT.

This fixes the following warning on Windows with latest Clang:
```
[160/3057] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Signals.cpp.obj
In file included from C:/git/llvm-project/llvm/lib/Support/Signals.cpp:260:
C:/git/llvm-project/llvm/lib/Support/Windows/Signals.inc(834,15): warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
  if (RetCode == (0xE0000000 | EX_IOERR))
      ~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~
1 warning generated.```