Fix a couple -Wdeprecated-copy issues

Building GDB with current git (future 13) Clang runs into these two
issues:

#1:

 src/gdb/symtab.h:1139:3: error: definition of implicit copy assignment operator for 'symbol' is deprecated because it has a user-declared copy constructor [-Werror,-Wdeprecated-copy]
   symbol (const symbol &) = default;
   ^

#2:

 src/gdb/dwarf2/read.c:834:23: error: definition of implicit copy constructor for 'partial_die_info' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
     partial_die_info& operator=(const partial_die_info& rhs) = delete;
		       ^

Fix them by adding the explicit defaulted versions of copy ctor and
copy-assign op appropriately.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* dwarf2/read.c (struct partial_die_info): Add defaulted copy
	ctor.
	* symtab.h (struct symbol): Add defaulted copy assignment
	operator.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2021-06-07  Pedro Alves  <pedro@palves.net>
+
+	* dwarf2/read.c (struct partial_die_info): Add defaulted copy
+	ctor.
+	* symtab.h (struct symbol): Add defaulted copy assignment
+	operator.
+
 2021-06-07  Pedro Alves  <pedro@palves.net>
 
 	* completer.c (RL_QF_SINGLE_QUOTE, RL_QF_DOUBLE_QUOTE)

gdb/dwarf2/read.c

@@ -832,6 +832,7 @@ struct partial_die_info : public allocate_on_obstack
     /* Disable assign but still keep copy ctor, which is needed
        load_partial_dies.   */
     partial_die_info& operator=(const partial_die_info& rhs) = delete;
+    partial_die_info (const partial_die_info &) = default;
 
     /* Adjust the partial die before generating a symbol for it.  This
        function may set the is_external flag or change the DIE's

gdb/symtab.h

@@ -1137,6 +1137,7 @@ struct symbol : public general_symbol_info, public allocate_on_obstack
     }
 
   symbol (const symbol &) = default;
+  symbol &operator= (const symbol &) = default;
 
   /* Data type of value */