Add ability to report when a variable's value is uninitialized,

based on information provided by the compiler.  Also add new
DWARF OP, DW_OP_GNU_uninit, for this purpose.

gdb/dwarf2loc.c

@@ -256,6 +256,8 @@ dwarf2_evaluate_loc_desc (struct symbol *var, struct frame_info *frame,
       VALUE_ADDRESS (retval) = address;
     }
 
+  set_value_initialized (retval, ctx->initialized);
+
   free_dwarf_expr_context (ctx);
 
   return retval;