Do not ignore by clauses for assign such that statements (#6026)

Fixes #6024

### What was changed?
- Do not ignore by clauses for assign such that statements

### How has this been tested?
- Extended an existing CLI test

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/DafnyCore/Dafny.atg

@@ -2391,19 +2391,16 @@ AssignStatement<out Statement/*!*/ s>
        s = new AssignSuchThatStmt(rangeToken, lhss, suchThat, suchThatAssume, null);
      } else if (exceptionRhs != null) {
        s = new AssignOrReturnStmt(rangeToken, lhss, exceptionRhs, keywordToken, rhss);
-       if (proof != null) {
-         s = new BlockByProofStmt(rangeToken, proof, s);
-       }
      } else {
        if (lhss.Count == 0 && rhss.Count == 0) {
          s = new BlockStmt(rangeToken, new List<Statement>()); // error, give empty statement
        } else {
          s = new AssignStatement(rangeToken, lhss, rhss);
-         if (proof != null) {
-           s = new BlockByProofStmt(rangeToken, proof, s);
-         }
        }
      }
+     if (proof != null) {
+       s = new BlockByProofStmt(rangeToken, proof, s);
+     }
   .)
   .
 

Source/IntegrationTests/TestFiles/LitTests/LitTest/ast/statement/assignSuchThat.dfy

@@ -5,4 +5,12 @@ method ByClause(b: bool) {
   var r: int :| false by {
     assume {:axiom} false;
   }
+}
+
+function F(x: int): int
+method ByClauseSeparateAssignment() {
+  var a;
+  a :| F(a) == 2 by {
+    assume {:axiom} F(10) == 2;
+  }
 }

Source/IntegrationTests/TestFiles/LitTests/LitTest/ast/statement/assignSuchThat.dfy.expect

@@ -1,2 +1,2 @@
 
-Dafny program verifier finished with 1 verified, 0 errors
+Dafny program verifier finished with 2 verified, 0 errors

docs/dev/news/6024.fix

@@ -0,0 +1 @@
+By clauses for assign-such-that statements (:|), are now never ignored.