[PCH] In HeaderFileInfoTrait::EqualKey(), use FileManager::getFile() to...

[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.

llvm-svn: 176450