[lldb] Use is_style_posix() for path style checks

Since a8b54834, there are two
distinct Windows path styles, `windows_backslash` (with the old
`windows` being an alias for it) and `windows_slash`.
4e4883e1 added helpers for
inspecting path styles.

The newly added windows_slash path style doesn't end up used in
LLDB yet anyway, as LLDB is quite decoupled from most of
llvm::sys::path and uses its own FileSpec class. To take it in
use, it could be hooked up in `FileSpec::Style::GetNativeStyle`
(in lldb/source/Utility/FileSpec.cpp) just like in the `real_style`
function in llvm/lib/Support/Path.cpp in
df0ba47c.

It is not currently clear whether there's a real need for using
the Windows path style with forward slashes in LLDB (if there's any
other applications interacting with it, expecting that style), and
what other changes in LLDB are needed for that to work, but this
at least makes some of the checks more ready for the new style,
simplifying code a bit.

Differential Revision: https://reviews.llvm.org/D113255