[flang] Format label scope is independent of block scope

Compilation of the following program currently generates a warning message:

        i = 1
        if (i .eq. 0) then
          write(6, 200) i
200       format (I8)
        end if
        write(6, 200) i
      end

x.f90:6:9: Label '200' is not in scope
          write(6, 200) i
          ^^^^^^^^^^^^^^^

Whereas branch targets must conform to the Clause 11.1.2.1 program
requirement "Transfer of control to the interior of a block from
outside the block is prohibited, ...", this doesn't apply to format
statement references.