[clang-tools-extra] Adopt FileManager's error-returning APIs

The FileManager has been updated to return llvm::ErrorOr from getFile
and getDirectory, this commit updates all the callers of those APIs from
clang.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@367617 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed