[FileSystem] Add convenience method to check for directories.

Replace calls to LLVM's is_directory with calls to LLDB's FileSytem
class. For this I introduced a new convenience method that, like the
other methods, takes either a path or filespec. This still uses the LLVM
functions under the hood.

Differential revision: https://reviews.llvm.org/D54135

llvm-svn: 346375
diff --git a/lldb/source/Core/ModuleList.cpp b/lldb/source/Core/ModuleList.cpp
index baa5ff5..9cff6c8 100644
--- a/lldb/source/Core/ModuleList.cpp
+++ b/lldb/source/Core/ModuleList.cpp
@@ -859,7 +859,7 @@
       auto search_path_spec = module_search_paths_ptr->GetFileSpecAtIndex(idx);
       FileSystem::Instance().Resolve(search_path_spec);
       namespace fs = llvm::sys::fs;
-      if (!fs::is_directory(search_path_spec.GetPath()))
+      if (!FileSystem::Instance().IsDirectory(search_path_spec))
         continue;
       search_path_spec.AppendPathComponent(
           module_spec.GetFileSpec().GetFilename().AsCString());