win: Pass /W4 in front of all the -wd flags.

This should fix many many -Wunused-parameter warnings in self-host builds on
Windows after r255382.  cl.exe doesn't care about the order of /W4 and
/wd flags, but clang-cl currently does (just like -Wno-foo -Wall order
matters for clang).  We might want to change how clang-cl behaves in
the future, but until then this change makes self-host builds much more
silent.

llvm-svn: 256315