[CSSimplify] Report unresolved base failure if leading-dot syntax base is a placeholder
If base type of the leading-dot syntax expression has been determined
to be a placeholder, this could only mean that `.<name>` couldn't be
resolved in the current context.
Resolves: rdar://104302974
(cherry picked from commit 9308796c66f2017db9280bdf114885678f2b9304)