gdb: fix pretty printing max depth behaviour

gdb: fix pretty printing max depth behaviour

The 'print max-depth' feature incorrectly causes GDB to skip printing
the string representation of pretty printed variables if the variable
is stored at a nested depth corresponding to the set max-depth value.
This change ensures that it is always printed before checking whether
the maximum print depth has been reached.

Regression tested with GCC 7.3.0 on x86_64, ppc64le, aarch64.

gdb/ChangeLog:

	* cp-valprint.c (cp_print_value): Replaced duplicate code.
	* guile/scm-pretty-print.c (ppscm_print_children): Check max_depth
	just before printing child values.
	(gdbscm_apply_val_pretty_printer): Don't check max_depth before
	printing string representation.
	* python/py-prettyprint.c (print_children): Check max_depth just
	before printing child values.
	(gdbpy_apply_val_pretty_printer): Don't check max_depth before
	printing string representation.

gdb/testsuite/ChangeLog:

	* gdb.python/py-format-string.c: Added a variable to test.
	* gdb.python/py-format-string.exp: Check string representation is
	printed at appropriate max_depth settings.
	* gdb.python/py-nested-maps.exp: Likewise.
	* gdb.guile/scm-pretty-print.exp: Add additional tests.