[flang] Block containing an interface

Name mangling may be invoked for an interface procedure contained in
a block in a context that does not have access to block ID mapping.
Procedures can't be defined inside a block, so name mangling doesn't
need a block map. Relax an assert to account for this.

    block
      interface
        subroutine ss(n) bind(c)
          integer :: n
        end subroutine
      end interface
      call ss(5)
    end block
    end