Use common_val_print in c-valprint.c

Use common_val_print in c-valprint.c

This changes c_value_print to call common_val_print.  This is more
complicated than the usual sort of common_val_print change, due to the
handling of RTTI.

gdb/ChangeLog
2020-03-13  Tom Tromey  <tom@tromey.com>

	* c-valprint.c (c_value_print): Use common_val_print.

gdb/testsuite/ChangeLog
2020-03-13  Tom Tromey  <tom@tromey.com>

	* gdb.base/printcmds.exp (test_print_strings): Add regression
	test.
	* gdb.base/printcmds.c (charptr): New typedef.
	(teststring2): New global.