Move the subfile stack to buildsym_compunit

Move the subfile stack to buildsym_compunit

This moves the global subfile_stack to be a member of
buildsym_compunit.  It also change this to be a std::vector, which
simplifies the code.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* buildsym.h (push_subfile, pop_subfile): Update declarations.
	* buildsym.c (struct buildsym_compunit) <m_subfile_stack>: New
	member.
	(struct subfile_stack): Remove.
	(subfile_stack): Remove.
	(push_subfile, pop_subfile, buildsym_init): Update.