[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/ModuleInterface/preconcurrency_conformances.swift

@@ -24,8 +24,8 @@
 // RUN: %target-swift-typecheck-module-from-interface(%t/Client.swiftinterface) -I %t -module-name Client \
 // RUN:   -target %target-swift-5.1-abi-triple -enable-upcoming-feature DynamicActorIsolation -verify
 
-// REQUIRES: asserts
 // REQUIRES: concurrency
+// REQUIRES: swift_feature_DynamicActorIsolation
 
 //--- A.swift
 public protocol P {