This changes the representation of closures in Zir and Sema. Rather than
a pair of instructions `closure_capture` and `closure_get`, the system
now works as follows:
* Each ZIR type declaration (`struct_decl` etc) contains a list of
captures in the form of ZIR indices (or, for efficiency, direct
references to parent captures). This is an ordered list; indexes into
it are used to refer to captured values.
* The `extended(closure_get)` ZIR instruction refers to a value in this
list via a 16-bit index (limiting this index to 16 bits allows us to
store this in `extended`).
* `Module.Namespace` has a new field `captures` which contains the list
of values captured in a given namespace. This is initialized based on
the ZIR capture list whenever a type declaration is analyzed.
This change eliminates `CaptureScope` from semantic analysis, which is a
nice simplification; but the main motivation here is that this change is
a prerequisite for #18816.
a6ca20b9a1