Use new and delete for struct infcall_suspend_state

Use new and delete for struct infcall_suspend_state

This changes infrun.c to use new and delete for infcall_suspend_state.
This enables the coming cleanups.

gdb/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* gdbthread.h (struct thread_suspend_state): Add initializers.
	(class thread_info) <suspend>: Remove initializer.
	* infrun.c (struct infcall_suspend_state): Add initializers.
	(save_infcall_suspend_state): Use new.
	(discard_infcall_suspend_state): Use delete.