GDScript: do not warn of return value discarded for super() inside _init()

DO NOT BATCH MERGE WITH #77324, WILL RESULT IN BROKEN CI

Currently, calling super() inside _init() throws a
RETURN_VALUE_DISCARDED warning. The analyzer identifies super() as being a
constructor, which therefore returns an object of the relevant class.
However, super() isn't really a constructor by itself: in this case, it
is _part_ of the constructor, and so doesn't "return" a value.

A test case for this is already in #77324, which contains the warning. I
am duplicating it here, without the warning, and it should conflict with
the other PR.

modules/gdscript/tests/scripts/analyzer/features/virtual_super_implemented.gd

@@ -0,0 +1,10 @@
+class TestOne:
+	func _init():
+		pass
+
+class TestTwo extends TestOne:
+	func _init():
+		super()
+
+func test():
+	pass

modules/gdscript/tests/scripts/analyzer/features/virtual_super_implemented.out

@@ -0,0 +1 @@
+GDTEST_OK