ExprCategory for guaranteed-in-place initializing expressions (#6623)

The primary change in this PR is to split the `Initializing` expression
category into separate `ReprInitializing` and `InPlaceInitializing`
categories, depending on whether initialization uses the types
initializing representation, or is guaranteed to be in place. It also
rationalizes and documents the SemIR-level semantics of those categories
(including where #5545's "ephemeral entire reference" category will
fit), and introduces two new inst kinds to close gaps exposed in the
process.

Some additional secondary changes:
- Consistently format the storage arguments of initializers with `to`,
regardless of whether initialization is in-place, and document the `to`
notation.
- Rename some inst kinds and functions, and restructure some of the
code, for clarity and consistency with the new documentation.
- Resolve a TODO to handle more category conversions in
`CategoryConverter`, in order to make it easier to reason about category
conversions.

See #6588 and the review history of this PR for background.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/cpp/thunk.cpp

@@ -731,10 +731,10 @@ auto PerformCppThunkCall(Context& context, SemIR::LocId loc_id,
 
   // Produce the result of the call, taking the value from the return storage.
   if (thunk_takes_return_address) {
-    result_id = AddInst<SemIR::InPlaceInit>(context, loc_id,
-                                            {.type_id = return_type_id,
-                                             .src_id = result_id,
-                                             .dest_id = return_slot_id});
+    result_id = AddInst<SemIR::MarkInPlaceInit>(context, loc_id,
+                                                {.type_id = return_type_id,
+                                                 .src_id = result_id,
+                                                 .dest_id = return_slot_id});
   }
 
   return result_id;