[clang] Change FileManager to use llvm::ErrorOr instead of null on failure

Summary:
Currently, clang's FileManager uses NULL as an indicator that a particular file
did not exist, but would not propagate errors like permission issues. Instead,
teach FileManager to use llvm::ErrorOr internally and return rich errors for
failures.

Reviewers: arphaman, bruno, martong, shafik

Subscribers: nemanjai, kbarton, MaskRay, jkorous, dexonsmith, kadircet, jsji, cfe-commits, lldb-commits

Tags: #clang, #lldb

Differential Revision: https://reviews.llvm.org/D65534

llvm-svn: 367618