AST: repair the build after swiftlang#39010

There is a reference to `std::string` being introduced without the necessary header being included.  Add the missing include.

Author: compnerd

include/swift/AST/USRGeneration.h

@@ -21,6 +21,8 @@
 
 #include "swift/Basic/LLVM.h"
 
+#include <string>
+
 namespace swift {
 class Decl;
 class AbstractStorageDecl;