[PATCH,c++] fix PR objc++/48479, ICE in cxx_mark_addressable

[Nathan Froyd]

My recent patch removing DECL_RTL from CONST_DECLs caused regressions in
the ObjC++ testsuite on Darwin targets.  The problem is that
DECL_REGISTER was being called on CONST_DECLs; DECL_REGISTER says:

/* In VAR_DECL and PARM_DECL nodes, nonzero means declared `register'.  */
#define DECL_REGISTER(NODE) (DECL_WRTL_CHECK (NODE)->decl_common.decl_flag_0)

Previously, the DECL_WRTL_CHECK was succeeding when given CONST_DECLs;
it no longer does.

The suggested fix is to simply move the CONST_DECL case in
cxx_mark_addressable; since DECL_REGISTER would have always returned
false for CONST_DECLs, there's no change in functionality.  Fixing
DECL_REGISTER to accurately reflect its comment would be helpful, but
there are other ICEs to fix if DECL_REGISTER only takes PARM_DECL and
VAR_DECL; I thought it best to submit that as a separate fix, if at all.

Patch was tested on Darwin targets via IainS and Dominique and fixed the
regression. OK to commit?

-Nathan

gcc/cp/
        * typeck.c (cxx_mark_addressable) [CONST_DECL]: Mark addressable
        and return immediately.

@@ -5373,7 +5373,6 @@ cxx_mark_addressable (tree exp)
                    || DECL_EXTERNAL (x));
        /* Fall through.  */
 
-      case CONST_DECL:
       case RESULT_DECL:
        if (DECL_REGISTER (x) && !TREE_ADDRESSABLE (x)
            && !DECL_ARTIFICIAL (x))
@@ -5391,6 +5390,7 @@ cxx_mark_addressable (tree exp)
        TREE_ADDRESSABLE (x) = 1;
        return true;
 
+      case CONST_DECL:
       case FUNCTION_DECL:
        TREE_ADDRESSABLE (x) = 1;
        return true;