Remove a couple std::string uses in diagnostics. (#4421)

We can't completely remove std::string from diagnostics because it's
probably better to provide a string than something like a
StringLiteralValueId or NameId (because those may be opaque for someone
trying to present the diagnostic). So this change is just playing
whackamole on another couple things that could easily use the new format
providers.

toolchain/check/handle_struct.cpp

@@ -6,6 +6,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/handle.h"
+#include "toolchain/diagnostics/format_providers.h"
 
 namespace Carbon::Check {
 
@@ -70,7 +71,7 @@ auto HandleParseNode(Context& context, Parse::StructTypeFieldId node_id)
 
 static auto DiagnoseDuplicateNames(Context& context,
                                    SemIR::InstBlockId type_block_id,
-                                   llvm::StringRef construct) -> bool {
+                                   bool is_struct_type_literal) -> bool {
   auto& sem_ir = context.sem_ir();
   auto fields = sem_ir.inst_blocks().Get(type_block_id);
   Map<SemIR::NameId, SemIR::InstId> names;
@@ -80,12 +81,13 @@ static auto DiagnoseDuplicateNames(Context& context,
     auto result = names.Insert(field_inst.name_id, field_inst_id);
     if (!result.is_inserted()) {
       CARBON_DIAGNOSTIC(StructNameDuplicate, Error,
-                        "duplicated field name `{1}` in {0}", std::string,
-                        SemIR::NameId);
+                        "duplicated field name `{1}` in "
+                        "{0:struct type literal|struct literal}",
+                        BoolAsSelect, SemIR::NameId);
       CARBON_DIAGNOSTIC(StructNamePrevious, Note,
                         "field with the same name here");
       context.emitter()
-          .Build(field_inst_id, StructNameDuplicate, construct.str(),
+          .Build(field_inst_id, StructNameDuplicate, is_struct_type_literal,
                  field_inst.name_id)
           .Note(result.value(), StructNamePrevious)
           .Emit();
@@ -103,7 +105,8 @@ auto HandleParseNode(Context& context, Parse::StructLiteralId node_id) -> bool {
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::StructLiteralStart>();
   auto type_block_id = context.args_type_info_stack().Pop();
-  if (DiagnoseDuplicateNames(context, type_block_id, "struct literal")) {
+  if (DiagnoseDuplicateNames(context, type_block_id,
+                             /*is_struct_type_literal=*/false)) {
     context.node_stack().Push(node_id, SemIR::InstId::BuiltinError);
     return true;
   }
@@ -128,7 +131,8 @@ auto HandleParseNode(Context& context, Parse::StructTypeLiteralId node_id)
   CARBON_CHECK(refs_id != SemIR::InstBlockId::Empty,
                "{{}} is handled by StructLiteral.");
 
-  if (DiagnoseDuplicateNames(context, refs_id, "struct type literal")) {
+  if (DiagnoseDuplicateNames(context, refs_id,
+                             /*is_struct_type_literal=*/true)) {
     context.node_stack().Push(node_id, SemIR::InstId::BuiltinError);
     return true;
   }