[flang] Catch impure defined assignments in DO CONCURRENT

The semantic checking of DO CONCURRENT bodies looks only at the
parse tree, not the typed expressions produced from it, so it
misses calls to defined assignment subroutines that arise from
assignment statements that resolve via generic interfaces into
subroutine calls.  Extend the checking to peek into the typed
assignment operations left on the parse tree by semantics.

Differential Revision: https://reviews.llvm.org/D146585