Extract symbol-writing function from parsers

Extract symbol-writing function from parsers

I noticed that several parsers shared the same code to write a symbol
reference to an expression.  This patch factors this code out into a
new function.

Regression tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-02-05  Tom Tromey  <tom@tromey.com>

	* parser-defs.h (write_exp_symbol_reference): Declare.
	* parse.c (write_exp_symbol_reference): New function.
	* p-exp.y (variable): Use write_exp_symbol_reference.
	* m2-exp.y (variable): Use write_exp_symbol_reference.
	* f-exp.y (variable): Use write_exp_symbol_reference.
	* d-exp.y (PrimaryExpression): Use write_exp_symbol_reference.
	* c-exp.y (variable): Use write_exp_symbol_reference.