swift

30789245 - [ast] Add a new helper SILBoxType::withMutable to take in a SILBoxType and produce a new BoxType with mutated mutability of its fields.

[ast] Add a new helper SILBoxType::withMutable to take in a SILBoxType and produce a new BoxType with mutated mutability of its fields. This is properly prepared for multiple field boxes since we take in an initializer_list of fields/mutability changes. Given that I made this change to SILBoxType, I aligned the same API on SILLayout to also take an initializer_list of fields/mutability changes instead of just assuming a single field layout.