Fix an issue where the oneliner printing of variables would ignore custom formatting

Because vector types use their formats in special ways (i.e. children get generated based on them), this change by itself would cause a regression in printing vector types with some custom formats
Work around that issue by special casing vector types out of this format-passdown mode. I believe there is a more general feature to be designed in this space, but until I see more cases of interest, I am going to leave this as a special case

Fixes rdar://20810062

llvm-svn: 239873