Move the context stack to buildsym_compunit

Move the context stack to buildsym_compunit

This moves the context stack globals to be members of
buildsym_compunit, changing the type to a std::vector in the process.

Because the callers expect the context stack object to be valid after
being popped, at Simon's suggestion I've changed pop_context to return
the object rather than the pointer.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* coffread.c (coff_symtab_read): Update.
	* xcoffread.c (read_xcoff_symtab): Update.
	* dwarf2read.c (new_symbol): Update.
	(read_func_scope, read_lexical_block_scope): Update.
	* dbxread.c (process_one_symbol): Update.
	* buildsym.h (context_stack, context_stack_depth): Don't declare.
	(outermost_context_p): Remove macro.
	(outermost_context_p, get_current_context_stack)
	(get_context_stack_depth): Declare.
	(pop_context): Return struct context_stack.
	* buildsym.c (struct buildsym_compunit) <m_context_stack: New
	member.
	(context_stack_size): Remove.
	(INITIAL_CONTEXT_STACK_SIZE): Remove.
	(prepare_for_building, end_symtab_get_static_block)
	(augment_type_symtab, push_context): Update.
	(pop_context): Return struct context_stack.
	(outermost_context_p, get_current_context_stack)
	(get_context_stack_depth): New functions.
	(buildsym_init): Update.