Change library search methods to return Optional instead of ErrorOr.

Change library search methods to return Optional instead of ErrorOr.

These methods weren't really throwing errors.  The only error used
was that a file could not be found, which isn't really an error at all
as we are searching paths and libraries for a file.  All of the callers
also ignored errors and just used the returned path if one was available.

Changing to return Optional<StringRef> as that actually reflects what
we are trying to do here: optionally find a given path.

llvm-svn: 264979