clang; Try to get windows-seh-async-verify.cpp to pass on mac

On macOS, file paths start with /Users/..., which clang-cl interptrets
as the /U switch followed by a preprocessor macro name to undefine.

Put the filename after `--` to prevent this. For consistency, move %s
to the end of the regular `clang` lines (where this isn't needed) as
well.