[libc++] Explicitly set output directory for DLL

DLLs on Windows are treated as runtime targets. Explicitly set the
output directory for them, to be consistent with other platforms.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300206 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed