[flang] Diagnose invalid initializations

f18 current ignores attempts to initialize (with =expr) things
that are not objects, or allows meaningless initializations of
things that have mistakenly been promoted to be objects.
Fix by refusing to promote to objects names that have any
attributes that cannot be applied to objects, and then catch
data initializations of symbols that are not objects.

Differential Revision: https://reviews.llvm.org/D142766