[flang] Correct IsHostAssociated() to be true for BLOCK constructs

The predicate IsHostAssocited() was implemented in a way that would
return true only for cases of host association into a module or inner
subprogram.  Technically, the use of a name in a BLOCK construct
that is not declared therein is considered in the Fortran standard
to also be a form of host association, and this matters when doing
error checking on DATA statements.

Differential Revision: https://reviews.llvm.org/D130388