Use OPT_IN instead of EXPERIMENTAL in diagnostic names

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

@@ -19,10 +19,10 @@ package usage
 import api.*
 
 fun use1() {
-    <!EXPERIMENTAL_API_USAGE!>f<!>()
+    <!OPT_IN_USAGE!>f<!>()
 }
 
-val use2 = <!EXPERIMENTAL_API_USAGE!>f<!>()
+val use2 = <!OPT_IN_USAGE!>f<!>()
 
 // FILE: inline-usage.kt
 
@@ -31,20 +31,20 @@ package usage
 import api.*
 
 inline fun inlineUse1() {
-    <!EXPERIMENTAL_API_USAGE!>f<!>()
+    <!OPT_IN_USAGE!>f<!>()
 }
 
 inline var inlineUse2: Unit
     get() {
-        <!EXPERIMENTAL_API_USAGE!>f<!>()
+        <!OPT_IN_USAGE!>f<!>()
     }
     set(value) {
-        <!EXPERIMENTAL_API_USAGE!>f<!>()
+        <!OPT_IN_USAGE!>f<!>()
     }
 
 var inlineUse3: Unit
     inline get() {
-        <!EXPERIMENTAL_API_USAGE!>f<!>()
+        <!OPT_IN_USAGE!>f<!>()
     }
     @API
     inline set(value) {
@@ -63,23 +63,23 @@ package usage
 import api.*
 
 private inline fun privateInline1() {
-    <!EXPERIMENTAL_API_USAGE!>f<!>()
+    <!OPT_IN_USAGE!>f<!>()
 }
 
 internal inline fun privateInline2() {
-    <!EXPERIMENTAL_API_USAGE!>f<!>()
+    <!OPT_IN_USAGE!>f<!>()
 }
 
 private inline var privateInline3: Unit
     get() {
-        <!EXPERIMENTAL_API_USAGE!>f<!>()
+        <!OPT_IN_USAGE!>f<!>()
     }
     set(value) {
-        <!EXPERIMENTAL_API_USAGE!>f<!>()
+        <!OPT_IN_USAGE!>f<!>()
     }
 
 internal class InternalClass {
     inline fun privateInline4() {
-        <!EXPERIMENTAL_API_USAGE!>f<!>()
+        <!OPT_IN_USAGE!>f<!>()
     }
 }