Recognize rustc_args_required_const attribute

This doesn't handle rustc_args_required_const, but it does allow us to
recognize it as a valid attribute.

gcc/rust/ChangeLog:

	* util/rust-attribute-values.h
	(Attributes::RUSTC_ARGS_REQUIRED_CONST): New constexpr variable.
	* util/rust-attributes.cc (__definitions): New entry for
	RUSTC_ARGS_REQUIRED_CONST.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>

Author: powerboat9

gcc/rust/util/rust-attribute-values.h

@@ -89,6 +89,9 @@ class Attributes
   static constexpr auto &TEST = "test";
 
   static constexpr auto &SIMD_TEST = "simd_test";
+
+  static constexpr auto &RUSTC_ARGS_REQUIRED_CONST
+    = "rustc_args_required_const";
 };
 } // namespace Values
 } // namespace Rust

gcc/rust/util/rust-attributes.cc

@@ -88,6 +88,9 @@ static const BuiltinAttrDefinition __definitions[]
 
      {Attrs::RUSTC_LAYOUT_SCALAR_VALID_RANGE_START, CODE_GENERATION},
 
+     // TODO: be careful about calling functions marked with this?
+     {Attrs::RUSTC_ARGS_REQUIRED_CONST, CODE_GENERATION},
+
      {Attrs::PRELUDE_IMPORT, NAME_RESOLUTION},
 
      {Attrs::RUSTC_DIAGNOSTIC_ITEM, STATIC_ANALYSIS},