[clang][AST] Reduce some AST node size. (#142585)
This patch reduces the size of several AST nodes by moving some fields
into the free bitfield space in the base `Stmt` class:
* `CXXForRangeStmt`: 96 → 88 bytes
* `ChooseExpr`: 56 → 48 bytes
* `ArrayTypeTraitExpr`: 56 → 48 bytes
* `ExpressionTraitExpr`: 40 → 32 bytes
* `CXXFoldExpr`: 64 → 56 bytes
* `ShuffleExpr`: 40 → 32 bytes
* `PackIndexingExpr`: 48 → 40 bytes
There are no noticeable memory savings (`Expr/Stmt` memory usage
125,824,496 vs 125,826,336 bytes for `SemaExpr.cpp`) in my testing,
likely because these node types are not among the most common in typical
ASTs.
