[flang] Don't emit faulty warnings for illegal COMMON blocks

SAVE statements referencing COMMON block names are not allowed in BLOCK
constructs. If they occur, an error is correctly emitted, but then flang
gets confused by the illegal SAVE and produces a faulty warning. This
patch removes that warning.

Consider this piece of Fortran (from the test blockconstruct02.f90):

program  main
  real r, s, t
  common /argmnt2/ r, s, t
  block
    save /argmnt2/
  end block
end program

Here flang (in addition to the error about the illegal SAVE) emits a
portability warning saying that the two definitions of argmnt2 have
different size, which does not make much sense.

This patch is a prerequisite for D125804, which in turn will make
blockconstruct02.f90 test this patch.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D132403