Introduce expression::evaluate

This introduces a new method, expression::evaluate, and changes the
top-level expression-evaluation functions to use it.  Stack temporary
handling is moved into this new method, which makes sense because that
handling was only done when "*pos == 0".

This patch avoids some temporary regressions related to stack
temporary in the larger expression rewrite series.  I've pulled it out
separately because it seems like a reasonable change in its own right,
and because it's better to avoid making that series even longer.

Regression tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-02-18  Tom Tromey  <tom@tromey.com>

	* expression.h (struct expression) <evaluate>: Declare method.
	* eval.c (evaluate_subexp): Simplify.
	(expression::evaluate): New method.
	(evaluate_expression, evaluate_type): Use expression::evaluate.