Fix use after free in DiskFilesOrDirectories

Summary:
We copy the local variable `Resolved` into `Storage` to keep it around. However, we then still let the `SearchDir` ref point to `Resolved` which then is used to access the already freed memory later on. With this patch we point to `Storage` which doesn't get deleted after the current scope exits.

Discovered by memory sanitizer in the CompletionTest.DirCompletionUsername test.

Reviewers: zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D42346

llvm-svn: 323082