MI: Allow non-raw varobj evaluation

Make the MI variable object expression evaluation, with the
-var-evaluate-expression command, recursively call pretty printers, to
match the output of normal expression printing.

Consider the following code:

        struct Foo { int val; };
        struct Wrapper { Foo foo; };

        int main() {
                Wrapper w;
                w.foo.val = 23;
        }

and this pretty printer file:

        import gdb.printing

        class FooPrinter:
          def __init__(self, val):
            self.val = val
          def to_string(self):
            return "Foo" + str(self.val["val"])

        class WrapperPrinter:
          def __init__(self, val):
            self.val = val
          def to_string(self):
            return self.val["foo"]

        test_printer = gdb.printing.RegexpCollectionPrettyPrinter("test")
        test_printer.add_printer('Foo', '^Foo$', FooPrinter)
        test_printer.add_printer('Wrapper', '^Wrapper$', WrapperPrinter)

        gdb.printing.register_pretty_printer(None, test_printer)

Setting a breakpoint at the end of the function, we call the following commands:

        -enable-pretty-printing
        ^done

        -var-create var_w @ w
        ^done,name="var_w",numchild="0",value="{val = 23}",type="Wrapper",dynamic="1",has_more="0"
        -var-create var_w_foo @ w.foo
        ^done,name="var_w_foo",numchild="0",value="Foo23",type="Foo",dynamic="1",has_more="0"

        -var-evaluate-expression var_w
        ^done,value="{val = 23}"
        -var-evaluate-expression var_w_foo
        ^done,value="Foo23"

        -data-evaluate-expression w
        ^done,value="Foo23"
        -data-evaluate-expression w.foo
        ^done,value="Foo23"

So, in the -var-evaluate-expression var_w case, we print the "raw" value
of w.foo, while in the -data-evaluate-expression w case, we print the
pretty printed w.foo value. After this patch, all of the above print
"Foo23".

gdb/ChangeLog:

	* varobj.c (varobj_formatted_print_options): Allow recursive
	pretty printing if pretty printing is enabled.

gdb/testsuite/ChangeLog:

	* gdb.python/py-prettyprint.c
	(struct to_string_returns_value_inner,
	struct to_string_returns_value_wrapper): New.
	(main): Add tsrvw variable.
	* gdb.python/py-prettyprint.py (ToStringReturnsValueInner,
	ToStringReturnsValueWrapper): New classes.
	(register_pretty_printers): Register new pretty-printers.
	* gdb.python/py-prettyprint.exp (run_lang_tests): Test printing
	recursive pretty printer.
	* gdb.python/py-mi.exp: Likewise.