Add multi-thread to ObjcExportHeaderGenerator when processing packageFragments

[alfonsocv12]

After looking at the profile I found some sections that can be process in parallel to improve framework generation speeds

KT-82436

[SvyatoslavScherbina]

IIUC, no other part of the compiler uses kotlinx.coroutines. If this is the case, this change effectively adds a new dependency.
Can the parallelization be done using standard JDK means?

For example, another part of the K/N compiler uses java.util.concurrent.Executors:

kotlin/kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/driver/phases/TopLevelPhases.kt

Lines 243 to 260 in 3afe02c

	val executor = Executors.newFixedThreadPool(threadsCount)
	val thrownFromThread = AtomicReference<Throwable?>(null)
	val tasks = fragmentsList.zip(generationStates).map { (fragment, generationState) ->
	Callable {
	try {
	// Currently, it's not possible to initialize the correct thread on `PerformanceManager` creation
	// because new threads are spawned here when `fragment` with its `PerformanceManager` is already initialized.
	fragment.performanceManager?.initializeCurrentThread()
	runAfterLowerings(fragment, generationState)
	} catch (t: Throwable) {
	thrownFromThread.set(t)
	}
	}
	}
	executor.invokeAll(tasks.toList())
	executor.shutdown()
	executor.awaitTermination(1, TimeUnit.DAYS)
	thrownFromThread.get()?.let { throw it }

Maybe it is worth checking other parts of the Kotlin compiler to see what is common.

[alfonsocv12]

I think theres more places that are using them already. This search returns 17 build files with the dependency

[SvyatoslavScherbina]

ObjCExport header generation process is very order-specific. For example, depending on the translation order, the same declarations may get different names when there are clashes that need mangling.

IIUC, this PR changes the translation order (and therefore is a breaking change) and makes it non-deterministic (which is highly undesirable, as the compiler should produce the same output for the same inputs).

[alfonsocv12]

Hello @SvyatoslavScherbina I think this should solve the order problem WDYT?