Drop Experimental.Impact, simplify related code and tests

See https://github.com/Kotlin/KEEP/issues/95#issuecomment-383889404

Drop Experimental.changesMayBreak, Experimental.Impact, the concept of
signature/body usage, same module exemption. Make the majority of tests
single-module because there is now no difference in the checker between
usages from the same module or from another module

libraries/stdlib/experimental/kotlin/annotations/Experimental.kt

@@ -12,17 +12,14 @@ import kotlin.reflect.KClass
 
 /**
  * Signals that the annotated annotation class is a marker of an experimental API. Any declaration annotated with that marker is thus
- * considered an experimental declaration and its call sites must accept the experimental aspect of it either by using [UseExperimental],
+ * considered an experimental declaration and its call sites should accept the experimental aspect of it either by using [UseExperimental],
  * or by being annotated with that marker themselves, effectively causing further propagation of that experimental aspect.
  */
 @Target(ANNOTATION_CLASS)
 @Retention(BINARY)
 @SinceKotlin("1.3")
-@Suppress("ANNOTATION_CLASS_WITH_BODY", "ANNOTATION_CLASS_MEMBER", "ReplaceArrayOfWithLiteral")
-annotation class Experimental(
-    val level: Level = Level.ERROR,
-    val changesMayBreak: Array<Impact> = arrayOf(Impact.COMPILATION, Impact.LINKAGE, Impact.RUNTIME) // arrayOf, not [] because of KT-22578
-) {
+@Suppress("ANNOTATION_CLASS_MEMBER")
+annotation class Experimental(val level: Level = Level.ERROR) {
     /**
      * Severity of the diagnostic that should be reported on usages of experimental API which did not explicitly accept the experimental aspect
      * of that API either by using [UseExperimental] or by being annotated with the corresponding marker annotation.
@@ -33,46 +30,11 @@ annotation class Experimental(
         /** Specifies that an error should be reported on incorrect usages of this experimental API. */
         ERROR,
     }
-
-    /**
-     * Impact of the experimental API specifies what aspects may break after that API is changed.
-     */
-    enum class Impact {
-        /**
-         * Signifies that changes in this experimental API can cause compilation errors or warnings in the client code.
-         *
-         * Non-signature usages (inside a function body, variable initializer, default argument value, etc.) of compilation-affecting
-         * experimental API are allowed either if the containing declaration is annotated with the experimental annotation marker
-         * and thus propagates the experimental aspect to its clients, or if there's a [UseExperimental] annotation entry with
-         * the corresponding annotation marker somewhere above that usage in the parse tree. Signature usages of compilation-affecting
-         * experimental API always require propagation (as long as the experimental API is declared in another module).
-         */
-        COMPILATION,
-        /**
-         * Signifies that changes in this experimental API can cause linkage errors, i.e. exceptions at runtime
-         * if the client code was not recompiled after the change.
-         *
-         * Any usage of a linkage-affecting experimental API requires its containing declaration to be annotated with
-         * the corresponding annotation marker (except non-signature usages in annotation marker's module).
-         */
-        LINKAGE,
-        /**
-         * Signifies that changes in this experimental API can cause changes in runtime behavior, including exceptions at runtime.
-         *
-         * Any usage of a runtime-affecting experimental API requires its containing declaration to be annotated with
-         * the corresponding annotation marker (except non-signature usages in the annotation marker's module).
-         */
-        RUNTIME,
-    }
 }
 
 /**
- * Allows to use experimental API denoted by the given markers in the annotated file, declaration, or expression. Each of the given markers
- * must be an annotation class, whose impact ([Experimental.changesMayBreak]) is [Experimental.Impact.COMPILATION].
- * Any other given annotation classes have no effect and are ignored.
- *
- * Only allows non-signature usages of the experimental API, i.e. inside a function body, variable initializer, default argument value, etc.
- * (Usages in declaration signatures must be propagated by annotating the affected signature with the marker annotation itself.)
+ * Allows to use experimental API denoted by the given markers in the annotated file, declaration, or expression.
+ * If a declaration is annotated with [UseExperimental], its usages are **not** required to opt-in to that experimental API.
  */
 @Target(CLASS, PROPERTY, LOCAL_VARIABLE, VALUE_PARAMETER, CONSTRUCTOR, FUNCTION, PROPERTY_GETTER, PROPERTY_SETTER, EXPRESSION, FILE)
 @Retention(SOURCE)