Some features of this file doesn't seem to have ever been used under
Windows. E.g. the --sysroot flag when compiling with msvc doesn't exist.
Also, it was not supported for the user to provide a sysroot anyways.

This allows for picking clang-cl instead of clang.

This should be temporary while I don't combine the wrapper for all
platforms.

It's been a few years that lld is not experimental anymore.
The advantage of always using lld is that we don't need to make
assumptions about cross-compiling.

The library is not called stdc++, the same way libc++ is not called c++.

Because we now renamed "stdc++" to "libstdc++", we don't need to
hardcode the stdlib name inside the conditional blocks.