This patch fixes improper handling of comma operator expression in a 
struct field initializer as described in:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46921

Currently, function output_init_element () does not evaluate the left 
hand expression in a comma operator that's used for a struct initializer 
field if the right hand side is zero-sized. However, the left hand 
expression must be evaluated if it's found to have side effects (for 
example, a function call).

Patch was successfully bootstrapped and tested on x86_64-linux.

--Dave