[VFS] Emit an error when entry at root level uses a relative path.

Entries with only a filename prevent us from building a file system tree and
cause the assertion

> Assertion failed: (NewParentE && "Parent entry must exist"), function uniqueOverlayTree, file clang/lib/Basic/VirtualFileSystem.cpp, line 1303.

Entries with a relative path are simply not discoverable during header search.

rdar://problem/28990865

Reviewers: bruno, benlangmuir

Reviewed By: bruno

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D49518

llvm-svn: 339164