Sema: rework beginComptimePtrMutation

This comment is now deleted because the task is completed in this
commit:

```
// TODO: Update this to behave like `beginComptimePtrLoad` and properly check/use
// `container_ty` and `array_ty`, instead of trusting that the parent decl type
// matches the type used to derive the elem_ptr/field_ptr/etc.
//
// This is needed because the types will not match if the pointer we're mutating
// through is reinterpreting comptime memory.
```

The main strategy is to change the ComptimePtrMutationKit struct so that
instead of `val: *Value` it now returns a tagged union which can be one
of three possibilities:

 * The pointer type matches the actual comptime Value so a direct
   modification is possible. Before this commit, the implementation
   incorrectly assumed this was always the case.

 * In the case of needing to write through a reinterpreted pointer, a
   mutable base Value pointer is provided along with a byte offset
   pointing to the element value in virtual memory.

 * Otherwise, it means a compile error must be emitted because one or
   both of the types (the owner of the value, or the pointer type being
   used to write through) do not have a well-defined memory layout.

After calling beginComptimePtrMutation, the one callsite now switches on
this tagged union and does the appropriate thing. The main new logic is
for the second case, which involves pointer reinterpretation, which now
takes this strategy:

 1. write the base value to a memory buffer.
 2. perform the pointer store at the proper byte offset, thereby
    modifying a subset of the buffer.
 3. read the base value from the memory buffer, overwriting the old base
    value.

0 files changed, 0 insertions, 0 deletions