Revert "Do not automatically set warning flags if buildtype is 'plain'"

This reverts commit 8ee1c9a07a.

No rationale was given for this change prior to merging. After the fact
it was described as desired by distro packagers, however as a distro
packager I believe this commit hurts me.

From a distro packaging perspective, we primarily care about one thing:
previously building code should still build. Hence, -Werror is bad for
our use case. meson handles this via -D werror which is different from
-D warning_level and as long as the former is disabled, warnings in the
build do not cause harm (but may attract review attention for upstream
to fix).

buildtype is a completely unrelated concern, and the intention for
=plain is to disable debug or optimization settings that result in
codegen differences and thus different built artifacts. This must not
happen in distro builds because the *distributed programs* should
conform to policy settings.

Unfortunately, completely disabling warnings happens silently, and
cannot be overridden even if you really, really believe you know what
you're doing. It is thematically broken, since use of
add_project_arguments() to add more -W flags is not likewise ignored.
But if you try to add -Wall in that manner, meson will lecture you to
use warning_level which you cannot do. And if you have custom warning
flags which depend on options enabled by judicious use of -Wall via
default_options: 'warning_level=1', then you end up with generated
warnings complaining about your command line rather than your code, such
as:

cc1: warning: ‘-Wformat-y2k’ ignored without ‘-Wformat’ [-Wformat-y2k]
cc1: warning: ‘-Wformat-extra-args’ ignored without ‘-Wformat’ [-Wformat-extra-args]
cc1: warning: ‘-Wformat-zero-length’ ignored without ‘-Wformat’ [-Wformat-zero-length]
cc1: warning: ‘-Wformat-contains-nul’ ignored without ‘-Wformat’ [-Wformat-contains-nul]
cc1: warning: ‘-Wformat-security’ ignored without ‘-Wformat’ [-Wformat-security]

which then break the build with -Werror.

Throughout all this, a buildtype of "plain" does *not* disable -D
werror=true, which is the part where distro builds actually break down!

Users who both wish to disable debug/optimization codegen, *and* disable
warning commentary, are encouraged to do so by doing both, not by doing
one and having the other be assumed.

Fixes #7399

mesonbuild/backend/backends.py

@@ -670,7 +670,7 @@ class Backend:
         # we weren't explicitly asked to not emit warnings (for Vala, f.ex)
         if no_warn_args:
             commands += compiler.get_no_warn_args()
-        elif self.get_option_for_target('buildtype', target) != 'plain':
+        else:
             commands += compiler.get_warn_args(self.get_option_for_target('warning_level', target))
         # Add -Werror if werror=true is set in the build options set on the
         # command-line or default_options inside project(). This only sets the