fix(sem): crash due to ill-formed if AST (#933)

## Summary

Fix the compiler crashing when encountering ill-formed `nkIfStmt`/
`nkIfExpr` AST and the maximum error count is larger than 1.

## Details

* in `semIf`, create an error node for the ill-formed branch instead of
  creating and reporting a `Report`
* this enables proper `nkError` propagation, which disables the second
  pass, preventing the invalid AST from being indexed into (this caused
  either an `IndexDefect` or `FieldDefect`)
* in addition, this also removes another use of immediate reporting via
  `handleReport`/`localReport`

---------

Co-authored-by: zerbina <100542850+zerbina@users.noreply.github.com>

compiler/sem/semstmts.nim

@@ -164,9 +164,8 @@ proc semIf(c: PContext, n: PNode; flags: TExprFlags): PNode =
       if it[0].isError:
         hasError = true
     else:
-      semReportIllformedAst(
-        c.config, it,
-        "Expected one or two subnodes for if statement, but found " & $it.len)
+      hasError = true
+      result[i] = c.config.newError(it, PAstDiag(kind: adSemIllformedAst))
 
   if hasError:
     result = c.config.wrapError(result)

tests/lang_callable/macros/tif_ill_formed.nim

@@ -0,0 +1,17 @@
+discard """
+  description: '''
+    Ensure that ill-formed if-statement AST created by macros is detected
+  '''
+  cmd: "nim check --hints:off $options $file"
+  nimoutfull: "true"
+  action: reject
+"""
+import std/macros
+
+macro t1(): untyped =
+  result =
+    nnkIfStmt.newTree(
+      newNimNode(nnkElseExpr)) #[tt.Error
+                ^ illformed AST:  else: <<0th child missing for nkElseExpr >>]#
+
+t1()