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

compiler/testData/diagnostics/testsWithStdLib/experimental/bodyUsagesAndInline.kt

@@ -0,0 +1,83 @@
+// !DIAGNOSTICS: -NOTHING_TO_INLINE
+// !USE_EXPERIMENTAL: kotlin.Experimental
+// FILE: api.kt
+
+package api
+
+@Experimental(Experimental.Level.WARNING)
+annotation class API
+
+@API
+fun f() {}
+
+// FILE: usage.kt
+
+package usage
+
+import api.*
+
+fun use1() {
+    <!EXPERIMENTAL_API_USAGE!>f<!>()
+}
+
+val use2 = <!EXPERIMENTAL_API_USAGE!>f<!>()
+
+// FILE: inline-usage.kt
+
+package usage
+
+import api.*
+
+inline fun inlineUse1() {
+    <!EXPERIMENTAL_API_USAGE!>f<!>()
+}
+
+inline var inlineUse2: Unit
+    get() {
+        <!EXPERIMENTAL_API_USAGE!>f<!>()
+    }
+    set(value) {
+        <!EXPERIMENTAL_API_USAGE!>f<!>()
+    }
+
+var inlineUse3: Unit
+    inline get() {
+        <!EXPERIMENTAL_API_USAGE!>f<!>()
+    }
+    @API
+    inline set(value) {
+        f()
+    }
+
+@API
+inline fun inlineUse4() {
+    f()
+}
+
+// FILE: private-inline-usage.kt
+
+package usage
+
+import api.*
+
+private inline fun privateInline1() {
+    <!EXPERIMENTAL_API_USAGE!>f<!>()
+}
+
+internal inline fun privateInline2() {
+    <!EXPERIMENTAL_API_USAGE!>f<!>()
+}
+
+private inline var privateInline3: Unit
+    get() {
+        <!EXPERIMENTAL_API_USAGE!>f<!>()
+    }
+    set(value) {
+        <!EXPERIMENTAL_API_USAGE!>f<!>()
+    }
+
+internal class InternalClass {
+    inline fun privateInline4() {
+        <!EXPERIMENTAL_API_USAGE!>f<!>()
+    }
+}