Rust: Fix bad join

Before
```
Evaluated relational algebra for predicate TypeInference::MethodResolution::MethodCall.getTrait/0#dispred#fc13ba6e@914858bt with tuple counts:
          153112   ~2%    {2} r1 = SCAN `Operation::Operation.isOverloaded/3#f0e64084` OUTPUT In.0, In.1
          153112   ~2%    {2}    | STREAM DEDUP

           18807   ~0%    {2} r2 = JOIN `TypeInference::getCallExprTraitQualifier/1#c084fe9f` WITH TypeInference::MethodResolution::MethodCallCallExpr#6eae461f ON FIRST 1 OUTPUT Lhs.0, Lhs.1

        65859035   ~3%    {3} r3 = JOIN `_IndexExpr::Generated::IndexExpr#9975e37a_TypeInference::MethodResolution::MethodCallIndexExpr.isInM__#shared` WITH Trait::Generated::Trait#ecf50173 CARTESIAN PRODUCT OUTPUT Rhs.0, _, Lhs.0
        65859035   ~0%    {3}    | REWRITE WITH Out.1 := "core::ops::index::Index"
           11191   ~0%    {2}    | JOIN WITH `Addressable::Addressable.getCanonicalPath/0#dispred#6044348f#bb` ON FIRST 2 OUTPUT Lhs.2, Lhs.0

             671   ~0%    {1} r4 = JOIN IndexExpr::Generated::IndexExpr#9975e37a WITH `TypeInference::MethodResolution::MethodCallIndexExpr.isInMutableContext/0#dispred#8c8ad425` ON FIRST 1 OUTPUT Lhs.0
         3948835   ~2%    {3}    | JOIN WITH Trait::Generated::Trait#ecf50173 CARTESIAN PRODUCT OUTPUT Rhs.0, _, Lhs.0
         3948835   ~2%    {3}    | REWRITE WITH Out.1 := "core::ops::index::IndexMut"
             671   ~1%    {2}    | JOIN WITH `Addressable::Addressable.getCanonicalPath/0#dispred#6044348f#bb` ON FIRST 2 OUTPUT Lhs.2, Lhs.0

          183781   ~0%    {2} r5 = r1 UNION r2 UNION r3 UNION r4
                          return r5
```

After
```
Evaluated relational algebra for predicate TypeInference::MethodResolution::MethodCall.getTrait/0#dispred#fc13ba6e@1b4a55e3 with tuple counts:
        153112   ~2%    {2} r1 = SCAN `Operation::Operation.isOverloaded/3#f0e64084` OUTPUT In.0, In.1
        153112   ~2%    {2}    | STREAM DEDUP

         11191   ~0%    {2} r2 = JOIN `_IndexExpr::Generated::IndexExpr#9975e37a_TypeInference::MethodResolution::MethodCallIndexExpr.isInM__#shared` WITH Stdlib::IndexTrait#e80543a5 CARTESIAN PRODUCT OUTPUT Lhs.0, Rhs.0

         18807   ~0%    {2} r3 = JOIN `TypeInference::getCallExprTraitQualifier/1#c084fe9f` WITH TypeInference::MethodResolution::MethodCallCallExpr#6eae461f ON FIRST 1 OUTPUT Lhs.0, Lhs.1

           671   ~0%    {1} r4 = JOIN IndexExpr::Generated::IndexExpr#9975e37a WITH `TypeInference::MethodResolution::MethodCallIndexExpr.isInMutableContext/0#dispred#8c8ad425` ON FIRST 1 OUTPUT Lhs.0
           671   ~1%    {2}    | JOIN WITH Stdlib::IndexMutTrait#4d6c31bd CARTESIAN PRODUCT OUTPUT Lhs.0, Rhs.0

        183781   ~0%    {2} r5 = r1 UNION r2 UNION r3 UNION r4
                        return r5
```

Author: hvitved

rust/ql/lib/codeql/rust/frameworks/stdlib/Stdlib.qll

@@ -250,6 +250,19 @@ class IndexTrait extends Trait {
   }
 }
 
+/**
+ * The [`IndexMut` trait][1].
+ *
+ * [1]: https://doc.rust-lang.org/std/ops/trait.IndexMut.html
+ */
+class IndexMutTrait extends Trait {
+  pragma[nomagic]
+  IndexMutTrait() { this.getCanonicalPath() = "core::ops::index::IndexMut" }
+
+  /** Gets the `index_mut` function. */
+  Function getIndexMutFunction() { result = this.(TraitItemNode).getAssocItem("index_mut") }
+}
+
 /**
  * The [`Box` struct][1].
  *

rust/ql/lib/codeql/rust/internal/TypeInference.qll

@@ -1786,8 +1786,8 @@ private module MethodResolution {
 
     override Trait getTrait() {
       if this.isInMutableContext()
-      then result.getCanonicalPath() = "core::ops::index::IndexMut"
-      else result.getCanonicalPath() = "core::ops::index::Index"
+      then result instanceof IndexMutTrait
+      else result instanceof IndexTrait
     }
   }