Add @_objcImplementation attribute

Part of swiftlang/swift#60630, but safe to merge before the compiler-side implementation.

Author: beccadax

Sources/SwiftParser/gyb_generated/DeclarationAttribute.swift

@@ -63,6 +63,7 @@ extension Parser {
     case _objcRuntimeName = "_objcRuntimeName"
     case _staticInitializeObjCMetadata = "_staticInitializeObjCMetadata"
     case _restatedObjCConformance = "_restatedObjCConformance"
+    case _objcImplementation = "_objcImplementation"
     case _optimize = "_optimize"
     case _clangImporterSynthesizedType = "_clangImporterSynthesizedType"
     case _weakLinked = "_weakLinked"

Tests/SwiftParserTest/Attributes.swift

@@ -238,4 +238,15 @@ final class AttributeTests: XCTestCase {
         """
     )
   }
+
+  func testObjcImplementationAttribute() throws {
+    AssertParse("""
+      @_objcImplementation extension MyClass {
+        func fn() {}
+      }
+      @_objcImplementation(Category) extension MyClass {
+        func fn2() {}
+      }
+      """)
+  }
 }

gyb_syntax_support/AttributeKinds.py

@@ -393,7 +393,11 @@ def __init__(self, name, swift_name=None):
                   ABIStableToAdd,  ABIStableToRemove,  APIStableToAdd,  APIStableToRemove,
                   code=70),
   # NOTE: 71 is unused
-  # NOTE: 72 is unused
+    DeclAttribute('_objcImplementation', 'ObjCImplementation',
+                  OnExtension,
+                  UserInaccessible,
+                  ABIBreakingToAdd, ABIBreakingToRemove, APIBreakingToAdd, APIBreakingToRemove,
+                  code=72),
     DeclAttribute('_optimize', 'Optimize',
                   OnAbstractFunction,  OnSubscript,  OnVar,
                   UserInaccessible,