dialyzer: fix bug concerning compiler option 'warnings_as_errors'

A typical scenario: the type digraph() is used in R16 (compilation
with the option 'warnings_as_errors' results in no warning); then the
module is analyzed by a 17 Dialyzer where digraph() is obsolete, and
since the warning is turned into an error Dialyzer fails to load the
module.

Thanks to Michael Truog.
dialyzer: Ignore ERL_COMPILER_OPTIONS when compiling

If, for instance, 'warn_unused_import' is present in
ERL_COMPILER_OPTIONS then many files in Erlang/OTP (Kernel, STDLIB,
...) cannot be analyzed by Dialyzer since the option is not present
when compiling Erlang/OTP, but 'warnings_as_errors' is.

The case that ERL_COMPILER_OPTIONS contains 'warnings_as_errors' can
only be handled by ignoring ERL_COMPILER_OPTIONS.
dialyzer: do a minor re-factoring

A minor re-factoring and generalization.
Merge branch 'hb/dialyzer/warnings_as_errors/OTP-12225' into maint

* hb/dialyzer/warnings_as_errors/OTP-12225:
  dialyzer: do a minor re-factoring
  dialyzer: Ignore ERL_COMPILER_OPTIONS when compiling
  dialyzer: fix bug concerning compiler option 'warnings_as_errors'
[observer] Remove note about R15B nodes in user guide

A note in the user guide says "Only R15B nodes can be observed". This
note was written because the observer_backend module did not exist in
earlier releases. It should have said "R15B or later". However, now
that we are on OTP-17, we don't think the note is necessary anymore so
it is now removed.
Merge branch 'siri/observer/remove-doc-note/OTP-12078' into maint

* siri/observer/remove-doc-note/OTP-12078:
  [observer] Remove note about R15B nodes in user guide