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.

gdb/expression.h

@@ -120,6 +120,11 @@ struct expression
       return elts[0].opcode;
   }
 
+  /* Evaluate the expression.  EXPECT_TYPE is the context type of the
+     expression; normally this should be nullptr.  NOSIDE controls how
+     evaluation is performed.  */
+  struct value *evaluate (struct type *expect_type, enum noside noside);
+
   /* Language it was entered in.  */
   const struct language_defn *language_defn;
   /* Architecture it was parsed in.  */