fix #13737

[timotheecour]

• fix Internal compiler error when using the new variable pragmas #13737
  now gives sane error just like before new syntax for lvalue references: var b {.byaddr.} = expr #13508:

/Users/timothee/git_clone/nim/timn/tests/nim/all/t10396.nim(5, 1) template/generic instantiation from here
/Users/timothee/git_clone/nim/timn/tests/nim/all/t10396.nim(6, 9) Error: invalid pragma: byaddr
  var a {.byaddr.} = s[0]