add STRING_LITERAL macro back to nimbase.h for compatibility (#24957)

refs #24956, refs #24302

lib/nimbase.h

@@ -470,6 +470,13 @@ typedef char* NCSTRING;
 
 #define NIM_STRLIT_FLAG ((NU)(1) << ((NIM_INTBITS) - 2)) /* This has to be the same as system.strlitFlag! */
 
+/* unused in codegen after 2.2 but keep for compatibility: */
+#define STRING_LITERAL(name, str, length) \
+   static const struct {                   \
+     TGenericSeq Sup;                      \
+     NIM_CHAR data[(length) + 1];          \
+  } name = {{length, (NI) ((NU)length | NIM_STRLIT_FLAG)}, str}
+
 /* declared size of a sequence/variable length array: */
 #if defined(__cplusplus) && defined(__clang__)
 #  define SEQ_DECL_SIZE 1