[VFS] Remove 'ignore-non-existent-contents' attribute for YAML-based VFS.
'ignore-non-existent-contents' stopped working after r342232 in a way
that the actual attribute value isn't used and it works as if it is
always `true`.
Common use case for VFS iteration is iterating through files in umbrella
directories for modules. Ability to detect if some VFS entries point to
non-existing files is nice but non-critical. Instead of adding back
support for `'ignore-non-existent-contents': false` I am removing the
attribute, because such scenario isn't used widely enough and stricter
checks don't provide enough value to justify the maintenance.
rdar://problem/
45176119
Reviewers: bruno
Reviewed By: bruno
Subscribers: hiraditya, dexonsmith, sammccall, cfe-commits
Differential Revision: https://reviews.llvm.org/D53228
llvm-svn: 345212