This implements initializing expression semantics for structs and
tuples, following #2006 and discussions since.
Tuple and (and analogously, struct) literals are treated as having a
mixed expression category that is later resolved based on how the
literal is used, as either a tuple initializer or a tuple value, at
which point we create a `TupleInit` or `TupleValue` that represents the
formation of the tuple initializer or tuple value from the tuple
literal.
There's quite a lot of TODOs here, and the SemIR representation is still
not quite right, but this seems like a good place to checkpoint some
incremental progress.
Instead of modeling array initialization as a thin wrapper around tuple
initialization, handle it like a function call, with a return slot as
part of its input. This better matches how initialization via a call to
`ImplicitAs::Convert` will eventually work, and in particular lets us do
in-place initialization of arrays rather than always creating a
temporary.
Richard Smith