Allow let to not have value when using importc (#14258)

* Allow let to not have value when using importc

This allows a let statement with the `{.importc.}` pragma to not be
initialised with a value. This allows us to declare C constants as Nim
lets without putting the value in the Nim code (which can lead to
errors, and requires us to go looking for the value). Fixes #14253

* Proper fix and documentation + changelog entry

* Improve testcase with one from timotheecour

* Add test to verify it working with macros

changelog.md

@@ -122,11 +122,11 @@
       deallocShared(x.val)
       x.val = nil
   ```
-
 - getImpl() on enum type symbols now returns field syms instead of idents. This helps
   with writing typed macros. Old behavior for backwards compatiblity can be restored
   with command line switch `--useVersion:1.0`.
-
+- ``let`` statements can now be used without a value if declared with
+  ``importc``/``importcpp``/``importjs``/``importobjc``.
 - The keyword `from` is now usable as an operator.
 - Exceptions inheriting from `system.Defect` are no longer tracked with
   the `.raises: []` exception tracking mechanism. This is more consistent with the
@@ -152,7 +152,6 @@ proc mydiv(a, b): int {.raises: [].} =
   The reason for this is that `DivByZeroDefect` inherits from `Defect` and
   with `--panics:on` `Defects` become unrecoverable errors.
 
-
 ## Compiler changes
 
 - Specific warnings can now be turned into errors via `--warningAsError[X]:on|off`.