Introduce common_val_print_checked

Introduce common_val_print_checked

A (much) later patch will remove the call to value_check_printable
from common_val_print.  This will needed to preserve some details of
how optimized-out structures are printed.

However, doing this will also break dw2-op-out-param.exp.  Making the
change causes "bt" to print:

However, the test wants to see:

... operand2=<optimized out>

That is, a wholly-optimized out structure should not print its fields.

So, this patch introduces a new common_val_print_checked, which calls
value_check_printable first, and then arranges to use it in the one
spot that affects the test suite.

I was not completely sure if it would be preferable to change the
test.  However, I reasoned that, assuming this output was intentional
in the first place, in a backtrace space is at a premium and so this
is a reasonable approach.  In other spots calling common_val_print,
this behavior is probably unintended, or at least a "don't care".

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

	* valprint.h (common_val_print_checked): Declare.
	* valprint.c (common_val_print_checked): New function.
	* stack.c (print_frame_arg): Use common_val_print_checked.