Add a location to indirect imports. (#4098)

By adding an `ImportDecl` instruction, this creates something that can
be referenced through `ImportIRInst`.
packages/no_prelude/implicit_imports_entities.carbon is getting a test
of this (import_conflict and import_conflict_reverse).

Also re-packs ImportIR from 24 bytes to 16 on 64-bit, since I'm touching
everywhere that makes one anyways.

toolchain/lower/handle.cpp

@@ -507,6 +507,11 @@ auto HandleImplDecl(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
   FatalErrorIfEncountered(inst);
 }
 
+auto HandleImportDecl(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
+                      SemIR::ImportDecl inst) -> void {
+  FatalErrorIfEncountered(inst);
+}
+
 auto HandleImportRefUnloaded(FunctionContext& /*context*/,
                              SemIR::InstId /*inst_id*/,
                              SemIR::ImportRefUnloaded inst) -> void {