Make dwarf_expr_piece::pieces an std::vector

Make dwarf_expr_piece::pieces an std::vector

Change the manually managed array dwarf_expr_piece::piece with an
std::vector.  After passing the pieces array to allocate_piece_closure,
dwarf2_evaluate_loc_desc_full doesn't need that data anymore.  We can
therefore move the content of the vector to avoid copying it.

Reg-tested on the buildbot.

gdb/ChangeLog:

	* dwarf2expr.h (struct dwarf_expr_piece): Move up.
	(struct dwarf_expr_context) <n_pieces>: Remove.
	<pieces>: Change type to std::vector.
	* dwarf2expr.c (dwarf_expr_context::dwarf_expr_context): Adjust.
	(dwarf_expr_context::~dwarf_expr_context): Don't manually free
	pieces.
	(dwarf_expr_context::add_piece): Adjust.
	* dwarf2loc.c (struct piece_closure): Initialize fields.
	<n_pieces>: Remove.
	<pieces>: Change type to std::vector.
	(allocate_piece_closure): Adjust, change parameter to
	std::vector rvalue and std::move it to piece_closure.
	(rw_pieced_value): Adjust.
	(check_pieced_synthetic_pointer): Adjust.
	(indirect_synthetic_pointer): Adjust.
	(coerce_pieced_ref): Adjust.
	(free_pieced_value_closure):  Adjust.  Use delete to free
	piece_closure.
	(dwarf2_evaluate_loc_desc_full): Adjust.  std::move ctx.pieces
	to allocate_piece_closure.
	(dwarf2_loc_desc_get_symbol_read_needs): Adjust.