[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/API/SBPlatform.cpp b/lldb/source/API/SBPlatform.cpp
index 2c254b2..aedad87 100644
--- a/lldb/source/API/SBPlatform.cpp
+++ b/lldb/source/API/SBPlatform.cpp
@@ -366,7 +366,7 @@
     if (src.Exists()) {
       uint32_t permissions = FileSystem::Instance().GetPermissions(src.ref());
       if (permissions == 0) {
-        if (llvm::sys::fs::is_directory(src.ref().GetPath()))
+        if (FileSystem::Instance().IsDirectory(src.ref()))
           permissions = eFilePermissionsDirectoryDefault;
         else
           permissions = eFilePermissionsFileDefault;