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.
Continuing with #3070. Just a dir and file rename (mostly removing
prefixes, although for parse_tree_fuzzer and parse_tree_file_test I'm
dropping "tree" instead of "parse"). Everything in the parse dir should
be marked as a move.
Richard Smith
Jon Ross-Perkins