Enable async let without the experimental flag

Author: DougGregor

include/swift/AST/Attr.def

@@ -593,7 +593,7 @@ SIMPLE_DECL_ATTR(_specializeExtension, SpecializeExtension,
   105)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(async, Async,
-  DeclModifier | OnVar | OnFunc | ConcurrencyOnly |
+  DeclModifier | OnVar | OnFunc |
   ABIBreakingToAdd | ABIBreakingToRemove |
   APIBreakingToAdd | APIBreakingToRemove,
   106)

test/SourceKit/CodeComplete/complete_override.swift.response

@@ -10,6 +10,16 @@
       key.typerelation: source.codecompletion.typerelation.notapplicable,
       key.num_bytes_to_erase: 0
     },
+    {
+      key.kind: source.lang.swift.keyword,
+      key.name: "async",
+      key.sourcetext: "async",
+      key.description: "async",
+      key.typename: "",
+      key.context: source.codecompletion.context.none,
+      key.typerelation: source.codecompletion.typerelation.notapplicable,
+      key.num_bytes_to_erase: 0
+    },
     {
       key.kind: source.lang.swift.keyword,
       key.name: "class",