Enable use of Labels as Values feature in Clang when not wrapped by GCC

[SlugFiller]

Split off from #77233. Tested on top of #92316, using the benchmark:

@tool
extends EditorScript

func _run() -> void:
	static_run()

static func static_run() -> void:
	var time_before := Time.get_ticks_msec()

	var r : int
	for i in range(10000000):
		var next : int
		var first := 1
		var second := 2
		for c in range(40):
			next = first + second
			first = second
			second = next
		r = next
	print(r)

	var total_time := Time.get_ticks_msec() - time_before
	print("Time taken: " + str(total_time))

In dev build, no notable difference. But in release build with lto=full, times went down from 16-18 seconds, to 13 seconds, i.e. ~20% improvement in speed.

[akien-mga]

Looks good.

Now that I see the changes closer again, IIUC, this only makes a different for distributions of Clang which do not define __GNUC__. I believe both Linux and Apple versions of Clang do define __GNUC__, so it's probably meaningful only for MSVC clang-cl and the vanilla Clang for Windows you're adding support for in the other PR?

Either way, this is correct and good to merge.

[akien-mga]

Thanks!