* margnus1/dialyzer/fix_maps_opaque/ERL-249/PR-1161/OTP-13878:
  erl_bif_types: Properly unopaque maps:merge/2 args

erl_bif_types:type/5 was calling erl_types:map_pairwise_merge/3 directly
with its (potentially opaque) arguments, causing Dialyzer crashes.

Bug (ERL-249) reported and minimised test case provided by Felipe
Ripoll.

The test 'proper' takes some time.

Let it handle `dial_warning()` input as well, to match what `format_warning/2` expects.

Warnings about clauses that cannot match and are also compiler generated
are suppressed unless none of the clauses return.

This feature is useful for non-Erlang BEAM languages (such as Elixir)
that compile to Erlang and expand certain language constructs into case
statements. In that case, as long as the language construct can succeed,
these warnings are undesired and appear spurious to users that do not
check the Erlang code that their program expands into.

Thanks to José Valim for the test (slightly modified).

t_map/3 previously required callers to perform this normalisation, but
as t_from_form/5 would sometimes fail to do so, this requirement is
relaxed.

Bug (ERL-177) reported and shrunk by Luke Imhoff.