[test] Mark tests that use experimental/upcoming features as such

Find all the usages of `--enable-experimental-feature` or
`--enable-upcoming-feature` in the tests and replace some of the
`REQUIRES: asserts` to use `REQUIRES: swift-feature-Foo` instead, which
should correctly apply to depending on the asserts/noasserts mode of the
toolchain for each feature.

Remove some comments that talked about enabling asserts since they don't
apply anymore (but I might had miss some).

All this was done with an automated script, so some formatting weirdness
might happen, but I hope I fixed most of those.

There might be some tests that were `REQUIRES: asserts` that might run
in `noasserts` toolchains now. This will normally be because their
feature went from experimental to upcoming/base and the tests were not
updated.

test/Constraints/opened_existentials_feature.swift

@@ -1,6 +1,8 @@
 // RUN: %target-typecheck-verify-swift -enable-upcoming-feature ImplicitOpenExistentials
 // RUN: %target-typecheck-verify-swift -swift-version 6
 
+// REQUIRES: swift_feature_ImplicitOpenExistentials
+
 #if _runtime(_ObjC)
 @objc
 protocol X {}