[gdb] Add debug_{exp,val}

When debugging cc1 I heavily rely on simple one-parameter debug functions
that allow me to inspect a variable of a common type, like:
- debug_generic_expr
- debug_gimple_stmt
- debug_rtx
and I miss similar functions in gdb.

Add functions to dump variables of types 'value' and 'expression':
- debug_exp, and
- debug_val.

Tested on x86_64-linux, by breaking on varobj_create, and doing:
...
(gdb) call debug_exp (var->root->exp.get ())
&"Operation: OP_VAR_VALUE\n"
&" Block symbol:\n"
&"  Symbol: aaa\n"
&"  Block: 0x2d064f0\n"
(gdb)
...
and:
...
(gdb) call debug_val (value)
&"5"
(gdb)
...

gdb/expprint.c

@@ -65,6 +65,19 @@ dump_prefix_expression (struct expression *exp, struct ui_file *stream)
   exp->op->dump (stream, 0);
 }
 
+/* Meant to be used in debug sessions, so don't export it in a header file.  */
+extern void ATTRIBUTE_USED debug_exp (struct expression *exp);
+
+/* Print EXP.  */
+
+void
+ATTRIBUTE_USED
+debug_exp (struct expression *exp)
+{
+  exp->op->dump (gdb_stdlog, 0);
+  gdb_flush (gdb_stdlog);
+}
+
 namespace expr
 {