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/lower/function_context.cpp

@@ -266,8 +266,8 @@ auto FunctionContext::GetDebugLoc(SemIR::InstId inst_id) -> llvm::DebugLoc {
                                loc.column_number, di_subprogram_);
 }
 
-auto FunctionContext::FinishInit(TypeInFile type, SemIR::InstId dest_id,
-                                 SemIR::InstId source_id) -> void {
+auto FunctionContext::InitializeStorage(TypeInFile type, SemIR::InstId dest_id,
+                                        SemIR::InstId source_id) -> void {
   switch (GetInitRepr(type).kind) {
     case SemIR::InitRepr::None:
       break;