Thread.isMainThread unnecessarily marked noasync

[weissi]

swift-corelibs-foundation/Sources/Foundation/Thread.swift

Lines 369 to 370 in c28bf23

	@available(*, noasync)
	open var isMainThread: Bool {

Thread.isMainThread is an important debugging tool, it should absolutely be available in noasync contexts.

Sure, the work around is just

extension Thread {
    static var isThisTheMainThread: Bool {
        return Thread.isMainThread
    }
}

but we shouldn't need to write this code.

Related to #5139