Make FileEntry::getName() valid across calls to FileManager::getFile()

Because we may change the name of a FileEntry inside getFile, the name
returned by FileEntry::getName() could be destroyed.  This was causing a
use-after-free when searching the HeaderFileInfo on-disk hashtable for a
module or pch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217385 91177308-0d34-0410-b5e6-96231b3b80d8