Fix struct expression regression

Fix struct expression regression

The patch to change struct expression to use new introduced a
regression -- there is a spot that reallocates expressions that I
failed to update.

This patch rewrites this code to follow the new approach.  Now the
rewriting is done in place.

gdb/ChangeLog
2020-12-06  Tom Tromey  <tom@tromey.com>

	PR ada/26999
	* ada-lang.c (replace_operator_with_call): Rewrite.