[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/stmt/print/then_stmt.swift

@@ -1,7 +1,6 @@
 // RUN: %target-swift-frontend -print-ast -enable-experimental-feature ThenStatements %s 2>&1 | %FileCheck %s
 
-// Required for experimental features
-// REQUIRES: asserts
+// REQUIRES: swift_feature_ThenStatements
 
 func foo() -> Int {
   if .random() {

test/stmt/then_stmt.swift

@@ -1,10 +1,8 @@
 // RUN: %target-typecheck-verify-swift -enable-bare-slash-regex -disable-availability-checking -enable-experimental-feature ThenStatements
 
-// Required for experimental features
-// REQUIRES: asserts
-
 // Required for regex
 // REQUIRES: swift_swift_parser
+// REQUIRES: swift_feature_ThenStatements
 
 func then(_: Int = 0, x: Int = 0, fn: () -> Void = {}) {}
 

test/stmt/then_stmt_exec.swift

@@ -4,9 +4,7 @@
 // RUN: %target-run %t/a.out | %FileCheck %s
 
 // REQUIRES: executable_test
-
-// Required for experimental features
-// REQUIRES: asserts
+// REQUIRES: swift_feature_ThenStatements
 
 func testDeferIf(_ x: Bool) -> Int {
   defer {

test/stmt/typed_throws.swift

@@ -1,6 +1,6 @@
 // RUN: %target-typecheck-verify-swift -enable-experimental-feature FullTypedThrows
 
-// REQUIRES: asserts
+// REQUIRES: swift_feature_FullTypedThrows
 
 enum MyError: Error {
 case failed