Replace NodeId with a hybrid LocationId in SemIR diagnostics. (#3810)

The purpose of this change is to allow something such as a FunctionDecl
instruction to note an imported instruction as the "loc_id". Note that
doesn't occur here: this change is already very sweeping in edits. There
is no testdata affected, intended to show equivalent behavior.

We might want to consolidate NodeId references towards LocationId, but
if that's preferred, I'd still like to split it out. A lot of this just
piping through LocationId where it's a build error otherwise, enough
that imports should be able to start using it for diagnostics.
ValueStores are added but still unused -- just flushing out structure
for review.

Restructuring SemIRLocation is necessary to use LocationId this way. For
TokenOnly, it's not getting used in Parse, so I migrated it to Check and
it's now specific to SemIRLocation.

I also considered making LocationId reference an InstId (which would
need to be an ImportRef) instead of an ImportIRInstId. However, that
would've required import.cpp to add instructions for decls which are
reached during resolution -- we typically don't have an inst ready for
use. An extra inst is essentially 16 bytes in InstId's ValueStore + 4
bytes in LocationId's ValueStore, whereas this is 8 bytes per.

toolchain/check/handle_expr_statement.cpp

@@ -15,7 +15,7 @@ static auto HandleDiscardedExpr(Context& context, SemIR::InstId expr_id)
   // If we discard an initializing expression, convert it to a value or
   // reference so that it has something to initialize.
   auto expr = context.insts().Get(expr_id);
-  Convert(context, context.insts().GetNodeId(expr_id), expr_id,
+  Convert(context, context.insts().GetLocationId(expr_id), expr_id,
           {.kind = ConversionTarget::Discarded, .type_id = expr.type_id()});
 
   // TODO: This will eventually need to do some "do not discard" analysis.