When printing a value, allow the root value's name to be elided, without omiting the
names of child values.
At the API level, this adds `SetHideRootName()`, which joins the existing
`SetHideName()` function.
This functionality is used by `dwim-print` and `expression`.
Fixes an issue identified by @jgorbe in https://reviews.llvm.org/D145609.
Differential Revision: https://reviews.llvm.org/D146783