Replace `SemIR::Node::GetAsFoo` and `SemIR::Node::Foo::Make` with
`SemIR::Foo` class that represents a particular kind of node, with named
fields.
Rename `SemIR::IntegerLiteral` and `SemIR::RealLiteral` to
`IntegerValue` / `RealValue` to better reflect their purpose and avoid a
name collision with the corresponding `SemIR` node kinds.
Remove `NodeKind::Invalid` and the `SemIR::Node` default constructor
entirely, as they were not used for anything.
A `let` declaration is represented by a `bind_name` node in SemIR:
```carbon-semir
%b: i32 = bind_name "b", %a
```
Because `Check` encounters the pattern before it sees the value, we
first create the `bind_name` node with an unset value and don't add it
to the block. Then, once we've seen and converted the initializer, we
update the `bind_name` to have the value and add it to the current
block, after the initializer code.
Combine the initialization, implicit conversion, and value category
conversion functions into a single function.
This substantially reduces the duplication between these steps, and
ensures that we support the same set of conversions in all these
contexts. This also fixes some issues where we would not use the proper
value representation for tuples and structs after performing implicit
conversions.
Continuing along with #3070. Note this is just a file rename, with BUILD
edits; every file previously in semantics/ should show as moved (except
maybe BUILDs, which split).
Richard Smith
Jon Ross-Perkins