Initial support for builtin functions. (#3803)

For now, a builtin function is defined by specifying a string literal
initializer in a function declaration:

```carbon
fn MyBuiltin(a: i32) -> i32 = "builtin.name";
```

End-to-end support is included for a sample `"int.add"` builtin
performing integer addition, covering constant evaluation and code
generation.

The implementation here needs substantial refactoring before we'll be
ready to start adding more builtins. That refactoring work will be
coming next. This change is aiming to checkpoint some incremental
progress.

toolchain/lower/file_context.cpp

@@ -10,6 +10,7 @@
 #include "toolchain/lower/function_context.h"
 #include "toolchain/sem_ir/entry_point.h"
 #include "toolchain/sem_ir/file.h"
+#include "toolchain/sem_ir/function.h"
 #include "toolchain/sem_ir/inst.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
@@ -95,6 +96,11 @@ auto FileContext::BuildFunctionDecl(SemIR::FunctionId function_id)
     return nullptr;
   }
 
+  // Don't lower builtins.
+  if (function.builtin_kind != SemIR::BuiltinFunctionKind::None) {
+    return nullptr;
+  }
+
   const bool has_return_slot = function.return_slot_id.is_valid();
   auto implicit_param_refs =
       sem_ir().inst_blocks().Get(function.implicit_param_refs_id);