[VFS] Add 'ignore-non-existent-contents' field to YAML files

Add 'ignore-non-existent-contents' to tell the VFS whether an invalid path
obtained via 'external-contents' should cause iteration on the VFS to stop.

If 'true', the VFS should ignore the entry and continue with the next. Allows
YAML files to be shared across multiple compiler invocations regardless of
prior existent paths in 'external-contents'. This global value is overridable
on a per-file basis.

This adds the parsing and write test part, but use by VFS comes next.

Differential Revision: https://reviews.llvm.org/D23422

rdar://problem/27531549

llvm-svn: 278456