…eScopeWithSplicing'

Uses new overrideScope feature to override base LLVM packages
without risking leaking incorrect non-overridden packages
from the base LLVM version.

To achieve no rebuilds or new attrs:
- Explicit list of packages to expose in an inherit instead
  of the entire scope
- Noting where workarounds for lack of overrideScope should
  be removed in a followup such as re-attaching a
  replaceVars patch or disabling separate tblgen package,
  but are kept for now to avoid a rebuild
- Override compiler-rt to continue incorrectly using
  the base LLVM's libllvm. This was an accident in
  the prior version, overrideScope tries to help us
  by correcting this mistake.

To avoid a large formatting diff a small helper has
been introduced that allows an overrideScope call with a single
arg, convincing nixfmt not to indent the scope body further.