properly fold constants for dynlib pragma (#22575)

fixes #12929

(cherry picked from commit 6b955ac4af)

compiler/pragmas.nim

@@ -330,7 +330,7 @@ proc expectDynlibNode(c: PContext, n: PNode): PNode =
     # {.dynlib: myGetProcAddr(...).}
     result = c.semExpr(c, n[1])
     if result.kind == nkSym and result.sym.kind == skConst:
-      result = result.sym.astdef # look it up
+      result = c.semConstExpr(c, result) # fold const
     if result.typ == nil or result.typ.kind notin {tyPointer, tyString, tyProc}:
       localError(c.config, n.info, errStringLiteralExpected)
       result = newEmptyStrNode(c, n)