Introduce a gdb_ref_ptr specialization for struct value
struct value is internally reference counted and so, while it also has
some ownership rules unique to it, it makes sense to use a gdb_ref_ptr
when managing it automatically.
This patch removes the existing unique_ptr specialization in favor of
a reference-counted pointer. It also introduces two other
clarifications:
1. Rename value_free to value_decref, which I think is more in line
with what the function actually does; and
2. Change release_value to return a gdb_ref_ptr. This change allows
us to remove the confusing release_value_or_incref function,
primarily by making it much simpler to reason about the result of
release_value.
gdb/ChangeLog
2018-04-06 Tom Tromey <tom@tromey.com>