Canonicalize generated functions for Core witnesses (#7729)

Use a single `SemIR::Function` per `Core` interface method, whether it's
generated locally or imported. This prevents generating duplicate
functions, which lead to different types when the witness appears in a
`FacetValue` as part of a specific for a class.

We use a `CanonicalValueStore` of `GeneratedFunction` objects that allow
finding an existing FunctionId for a `Generated` special function before
(re-)generating it. Mangling for `Generated` functions is also moved to
use the values from the `GeneratedFunction`'s canonicalization key, so
that we have a consistent source of truth for the unique ID of a
`Generated` function across all files.

New tests are in
`toolchain/check/testdata/impl/custom_witness/destroy.carbon`.

toolchain/sem_ir/function.cpp

@@ -7,6 +7,7 @@
 #include <optional>
 #include <variant>
 
+#include "toolchain/base/canonical_value_store_impl.h"
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/base/value_store_impl.h"
 #include "toolchain/sem_ir/file.h"
@@ -153,6 +154,20 @@ auto DecomposeVirtualFunction(const File& sem_ir, InstId fn_decl_id,
           .specific_id = specific_id};
 }
 
+auto Function::GetBuiltinFunctionKind(const File& file) const
+    -> BuiltinFunctionKind {
+  switch (special_function_kind) {
+    case SpecialFunctionKind::Builtin:
+      return non_generated_builtin_function_kind();
+    case SpecialFunctionKind::Generated: {
+      auto generated_id = GeneratedFunctionId(special_function_kind_data.index);
+      return file.generated_functions().Get(generated_id).builtin_function_kind;
+    }
+    default:
+      return BuiltinFunctionKind::None;
+  }
+}
+
 auto Function::GetDeclaredReturnType(const File& file,
                                      SpecificId specific_id) const -> TypeId {
   if (!return_type_inst_id.has_value()) {
@@ -178,4 +193,7 @@ auto Function::GetDeclaredReturnForm(const File& file,
 namespace Carbon {
 template class ValueStore<SemIR::FunctionId, SemIR::Function,
                           Tag<SemIR::CheckIRId>>;
+template class CanonicalValueStore<
+    SemIR::GeneratedFunctionId, SemIR::GeneratedFunction::CanonicalKey,
+    Tag<SemIR::CheckIRId>, SemIR::GeneratedFunction>;
 }  // namespace Carbon