Start to preserve type sugar in diagnostics. (#7768)

Instead of always printing types as canonical, attempt to find a sugared
type where possible, and include that type in the diagnostic. We can
only do this when given the instruction whose type is being printed
(`TypeOfInstId`) rather than the canonical type ID.

Initial support here is intentionally minimal: just looking through
calls to the callee's declared return type, and looking through pointer
dereferences and corresponding pointer types, to build out the initial
infrastructure. More cases can be added later; this degrades gracefully
to using the canonical type if a better type can't be found.

Assisted-by: Claude Opus 5 via Antigravity

toolchain/sem_ir/stringify.cpp

@@ -19,6 +19,7 @@
 #include "toolchain/sem_ir/singleton_insts.h"
 #include "toolchain/sem_ir/specific_interface.h"
 #include "toolchain/sem_ir/struct_type_field.h"
+#include "toolchain/sem_ir/sugared_type.h"
 #include "toolchain/sem_ir/type_info.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
@@ -876,6 +877,10 @@ auto StringifyConstantInst(const File& sem_ir, InstId outer_inst_id)
   return Stringify(sem_ir, step_stack);
 }
 
+auto StringifyTypeOfInst(const File& sem_ir, InstId inst_id) -> std::string {
+  return StringifyConstantInst(sem_ir, GetSugaredTypeOfInst(sem_ir, inst_id));
+}
+
 auto StringifySpecific(const File& sem_ir, SpecificId specific_id)
     -> std::string {
   StepStack step_stack(&sem_ir);