[Support] Don't return an error if realPath fails.

In openFileForRead, we would not previously return an error
if real_path resolution failed.  After a recent patch, we
started propagating this error up.  This caused a failure
in clang when trying to call openFileForRead("nul").  This
patch restores the previous behavior of not propagating this
error up.

llvm-svn: 297488