[flang] add memory effects interface to (hl)fir.declare

These operations should have no memory effect, but doing so causes the
declare to be removed by dead code elimination if the result value is
unused. fir.declare is intended to be used to generate debug information
about variables. Debug information may still be desirable even about
unused variables, so we don't want to remove the declare operations when
performing dead code elimination.

Differential Revision: https://reviews.llvm.org/D157626