Use new and delete for values

Use new and delete for values

This adds a constructor and destructor to struct value, and then
changes value.c to use "new" and "delete".

While doing this I noticed a memory leak -- value_decref was not
freeing value::optimized_out.  This patch fixes this leak.

gdb/ChangeLog
2018-04-06  Tom Tromey  <tom@tromey.com>

	* value.c (struct value): Add constructor, destructor, and member
	initializers.
	(allocate_value_lazy, value_decref): Update.