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

compiler/semstmts.nim

@@ -531,8 +531,6 @@ proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
             # special type inference rule: 'var it = ownedPointer' is turned
             # into an unowned pointer.
             typ = typ.lastSon
-    else:
-      if symkind == skLet: localError(c.config, a.info, errLetNeedsInit)
 
     # this can only happen for errornous var statements:
     if typ == nil: continue
@@ -620,6 +618,9 @@ proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
           defaultConstructionError(c, v.typ, v.info)
         else:
           checkNilable(c, v)
+        # allow let to not be initialised if imported from C:
+        if v.kind == skLet and sfImportc notin v.flags:
+          localError(c.config, a.info, errLetNeedsInit)
       if sfCompileTime in v.flags:
         var x = newNodeI(result.kind, v.info)
         x.add result[i]