[llvm][Support] Fix missing field in WSADATA warning

```
[136/6185] Building CXX object lib\Support\CMakeFiles\LLVMSupport.dir\raw_ostream.cpp.obj
C:\Work\david.spickett\llvm-project\llvm\lib\Support\raw_ostream.cpp(977,23): warning: missing field 'wHighVersion' initializer [-Wmissing-field-initializers]
  977 |   WSADATA WsaData = {0};
      |                       ^
```

According to
https://learn.microsoft.com/en-us/windows/win32/api/winsock/ns-winsock-wsadata
this can have any number of fields depending on some ifdefs,
so I've replaced `{0}` with a memset of whatever the size happens to be.

Probably doesn't matter for correctness anyway, but the compiler
won't see it used uninitialised at least.