Add builtin for performing checked conversion between integer types. (#4523)

As a prerequisite for switching the type of int literals to be the
`IntLiteral` type, add support for performing conversions of in-bounds
integer constant values to other integer types in which they fit.

This incidentally is our first compile-time-only builtin function, so
add very minimal support for compile-time-only functions while we're
here.

toolchain/sem_ir/builtin_function_kind.cpp

@@ -202,6 +202,10 @@ constexpr BuiltinInfo FloatMakeType = {"float.make_type",
 constexpr BuiltinInfo BoolMakeType = {"bool.make_type",
                                       ValidateSignature<auto()->Type>};
 
+// Converts between integer types, with a diagnostic if the value doesn't fit.
+constexpr BuiltinInfo IntConvertChecked = {
+    "int.convert_checked", ValidateSignature<auto(AnyInt)->AnyInt>};
+
 // "int.snegate": integer negation.
 constexpr BuiltinInfo IntSNegate = {"int.snegate",
                                     ValidateSignature<auto(IntT)->IntT>};
@@ -373,4 +377,8 @@ auto BuiltinFunctionKind::IsValidType(const File& sem_ir,
   return ValidateFns[AsInt()](sem_ir, arg_types, return_type);
 }
 
+auto BuiltinFunctionKind::IsCompTimeOnly() const -> bool {
+  return *this == BuiltinFunctionKind::IntConvertChecked;
+}
+
 }  // namespace Carbon::SemIR