Clean up handling of Call params (#5061)

- Explicitly document that `*Param` and `*ParamPattern` insts represent
`Call` parameters.
- Stop wrapping compile-time parameter patterns in `ValueParamPattern`
insts (because they aren't `Call` parameters).
- Document how `MatchContext::results_` relates to the `Call`
parameters, and be more consistent about when it's written to.
- Remove `RuntimeParamIndex::Unknown`: we no longer need to distinguish
"this `Param`'s runtime index is unknown" from "this `Param` isn't a
runtime param", because we no longer use `Param`s at all in the latter
case.
- Rename `RuntimeParamIndex` to `CallParamIndex`.

As a side effect of removing the `ValueParamPattern` insts, this fixes a
minor diagnostic bug where `NoteInitializingParam` didn't identify the
specific parameter that led to a deduction failure, because it expects
generic parameters to only be represented by `SymbolicBindingPattern`s,
but before this change they could be wrapped in `ValueParamPattern`s.

toolchain/check/handle_function.cpp

@@ -69,7 +69,7 @@ auto HandleParseNode(Context& context, Parse::ReturnTypeId node_id) -> bool {
       context, node_id,
       {.type_id = type_id,
        .subpattern_id = return_slot_pattern_id,
-       .runtime_index = SemIR::RuntimeParamIndex::Unknown});
+       .index = SemIR::CallParamIndex::None});
   context.node_stack().Push(node_id, param_pattern_id);
   return true;
 }