[stdlib] Dictionary: Remove workaround for rdar://problem/44612356

Author: lorentey

stdlib/public/core/Dictionary.swift

@@ -918,8 +918,13 @@ extension Dictionary {
       return _variant.lookup(key) ?? defaultValue()
     }
     _modify {
-      let index = _variant.lookupOrInsert(key, default: defaultValue)
-      let address = _variant.asNative._values + index.bucket
+      let (index, found) = _variant.mutatingFind(key)
+      let native = _variant.asNative
+      if !found {
+        let value = defaultValue()
+        native._insert(at: index, key: key, value: value)
+      }
+      let address = native._values + index.bucket
       yield &address.pointee
       _fixLifetime(self)
     }
@@ -3544,19 +3549,6 @@ extension Dictionary._Variant {
     }
   }
 
-  @usableFromInline // FIMXE: Should be @inlinable (rdar://problem/44612356)
-  internal mutating func lookupOrInsert(
-    _ key: Key,
-    default defaultValue: () -> Value
-  ) -> _NativeDictionary<Key, Value>.Index {
-    let (index, found) = mutatingFind(key)
-    if !found {
-      let value = defaultValue()
-      asNative._insert(at: index, key: key, value: value)
-    }
-    return index
-  }
-
   /// Ensure uniquely held native storage, while preserving the given index.
   /// (If the variant had bridged storage, then the returned index will be the
   /// corresponding native representation. Otherwise it's kept the same.)