[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.

Change is done both in LLVM and Clang, corresponding Clang commit is r345212.

rdar://problem/45176119

Reviewers: bruno

Reviewed By: bruno

Subscribers: hiraditya, dexonsmith, sammccall, cfe-commits

Differential Revision: https://reviews.llvm.org/D53228

llvm-svn: 345213