Fix use-after-move in compile/compile-cplus-types.c

Fix use-after-move in compile/compile-cplus-types.c

Patch

  d82b3862f12 ("compile: Remove non-const reference parameters")

introduced a regression in compile/compile-cplus-types.c.  The new_scope
variable in compile_cplus_instance::enter_scope is used after it was
std::moved.  This patch fixes it by referring to the back of the vector
where it was moved instead.

gdb/ChangeLog:

	* compile/compile-cplus-types.c
	(compile_cplus_instance::enter_scope): Don't use new_scope after
	std::move.