[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/layout_pre_specialization.swift

@@ -5,6 +5,8 @@
 // RUN: %target-swift-typecheck-module-from-interface(%t/LayoutPrespec.swiftinterface) -module-name LayoutPrespec -enable-experimental-feature LayoutPrespecialization -disable-availability-checking
 // RUN: %FileCheck %s < %t/LayoutPrespec.swiftinterface
 
+// REQUIRES: swift_feature_LayoutPrespecialization
+
 // CHECK: @_specialize(exported: true, kind: full, where @_noMetadata A : _Class)
 // CHECK-NEXT: public func test<A>(a: A) -> A
 @_specialize(exported: true, where @_noMetadata A : _Class)