[clang] Adopt llvm::ErrorOr in FileManager methods

[clang] Adopt llvm::ErrorOr in FileManager methods

Previously, the FileManager would use NULL returns to signify whether a file existed, but that doesn’t cover permissions issues or anything else that might occur while trying to stat or read a file. Instead, convert getFile and getDirectory into returning llvm::ErrorOr

Signed-off-by: Harlan Haskins <harlan@apple.com>
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367615 91177308-0d34-0410-b5e6-96231b3b80d8