Use std::vector for "registers_used" in compile feature

Use std::vector for "registers_used" in compile feature

This changes the GDB compile code to use std::vector<bool> when
computing which registers are used.  This is a bit more idiomatic, but
the main benefit is that it also adds some checking when the libstd++
debug mode is enabled.

2021-01-23  Tom Tromey  <tom@tromey.com>

	* symtab.h (struct symbol_computed_ops) <generate_c_location>:
	Change type of "registers_used".
	* dwarf2/loc.h (dwarf2_compile_property_to_c): Update.
	* dwarf2/loc.c (dwarf2_compile_property_to_c)
	(locexpr_generate_c_location, loclist_generate_c_location): Change
	type of "registers_used".
	* compile/compile.h (compile_dwarf_expr_to_c)
	(compile_dwarf_bounds_to_c): Update.
	* compile/compile-loc2c.c (pushf_register_address)
	(pushf_register, do_compile_dwarf_expr_to_c)
	(compile_dwarf_expr_to_c, compile_dwarf_bounds_to_c): Change type
	of "registers_used".
	* compile/compile-c.h (generate_c_for_variable_locations):
	Update.
	* compile/compile-c-symbols.c (generate_vla_size)
	(generate_c_for_for_one_variable): Change type of
	"registers_used".
	(generate_c_for_variable_locations): Return std::vector.
	* compile/compile-c-support.c (generate_register_struct): Change
	type of "registers_used".
	(compute): Update.