[PCH] In HeaderFileInfoTrait::EqualKey(), use FileManager::getFile() to compare two filenames, instead of llvm::sys::fs::equivalent().

llvm::sys::fs::equivalent() does 2 stat calls every time it's called. Use FileManager::getFile() to take advantage
of the stat caching that FileManager is providing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176450 91177308-0d34-0410-b5e6-96231b3b80d8