[Sema][CodeComplete] Handle symlinks for include code completion

Summary:
Previously any symlinks would be ignored since the directory
traversal doesn't follow them.

With this change we now follow symlinks (via a `stat` call
in order to figure out the target type of the symlink if it
is valid).

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74790