iOS,macOS: Compile Swift with opt flags in opt builds (#191622)

We never passed optimisation flags to `swiftc`, and the compiler's
default is `-Onone`, so all of the Darwin embedders' Swift shipped
unoptimised in profile and release, and `assert()`s triggered in
shipping binaries, as seen in flutter/flutter#190909.

This updates our optimisation gn configs to set equivalent `swiftflags`
alongside the `cflags` we already set. For iOS that means `-Osize` (to
match `-Os` for Obj-C/C++). For macOS, that means `-O` (to match `-O2`
for Obj-C/C++, Swift just has -O, no -O2, -O3). For both, we set and
`-Onone` for `:no_optimize`.

When `is_debug` is true, we default to unopt. Swift `assert()` behaves
like `FML_DCHECK` (active in unopt, compiled out in opt builds) and we
should use `precondition()` where we'd use `FML_CHECK` -- i.e. cases we
want to assert even in release builds.

The following dev-specific asserts are now compiled out from shipped
engines:
* `DisplayLinkManager.shared` main-thread check
* `KeyboardInsetManager` vsync client internal-state check
* `FlutterRunLoop.mainRunLoop` the guard check already triggers
* `TraceScope.deinit` ensure `end()` was called

This also moves the main-thread check in
`FlutterRunLoop.ensureMainLoopInitialized` to a `precondition`. Binding
the main run loop to the wrong thread silently leaves everything later
scheduled on it running on the wrong thread so we should continue to
blow up in all modes.

This change chops 130272 bytes (0.79%) off `Flutter.framework` on
`ios_release`.

Issue: https://github.com/flutter/flutter/issues/191616
Related: https://github.com/flutter/flutter/issues/190909 (demonstrates
existing behaviour)

No new tests, since this is a compile config change: the build *is* the
test, along with not breaking the existing tests.

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