[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/Parse/validate_new_parser_diagnostics.swift

@@ -1,5 +1,5 @@
 // REQUIRES: swift_swift_parser
-// REQUIRES: asserts
+// REQUIRES: swift_feature_ParserValidation
 
 // Checks that skipping function bodies doesn't cause the new parser validation
 // to fail. This can currently be the case because the new parser doesn't