Refactor InstBlockStore's API, AddDefaultValue -> AddPlaceholder (#5166)

`AddDefaultValue` doesn't quite capture the intended semantics; it
should typically be replaced with an actual value when dealing with
control flows. Trying to indicate the "assign later" with
`AddPlaceholder`, mirroring `AddPlaceholderInst`.

Shifting the `protected` functionality on `BlockValueStore` so that it's
not providing functions just for `InstBlockStore` to use. Also hoping
that seeing the comments next to the function name makes them easier to
understand, whereas `using` buries that a little.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/subpattern.cpp

@@ -20,7 +20,7 @@ auto EndSubpatternAsExpr(Context& context, SemIR::InstId result_id)
     // will be determined later.
     AddInst(context,
             SemIR::LocIdAndInst::NoLoc<SemIR::Branch>(
-                {.target_id = context.inst_blocks().AddDefaultValue()}));
+                {.target_id = context.inst_blocks().AddPlaceholder()}));
   } else {
     // This single-block region will be inserted as a SpliceBlock, so we don't
     // need control flow out of it.