[ASTGen/Tests] Add 'REQUIRES: string_processing'

This test imports _StringProcessing implicitly

rdar://159635602

test/ASTGen/attrs_objc.swift

@@ -11,6 +11,7 @@
 // REQUIRES: swift_swift_parser
 // REQUIRES: objc_interop
 // REQUIRES: swift_feature_ParserASTGen
+// REQUIRES: string_processing
 
 @objc class MyCls {
   @objc(theValue) var value: Int { 1 }