gdb: constify parameter of value_copy

In a following patch, I have a const value I want to copy using a
value_copy.  However, value_copy takes a non-const source value, at the
moment.  Change the paramter to be const,

If the source value is not lazy, we currently call
value_contents_all_raw, which calls allocate_value_contents, to get a
view on the contents.  They both take a non-const value, that's a
problem.  My first attempt at solving it was to add a const version of
value_contents_all_raw, make allocate_value_contents take a const value,
and either:

 - make value::contents mutable
 - make allocate_value_contents cast away the const

The idea being that allocating the value contents buffer does modify the
value at the bit level, but logically that doesn't change its state.

That was getting a bit complicated, so what I ended up doing is make
value_copy not call value_contents_all_raw.  We know at this point that
the value is not lazy, so value::contents must have been allocate
already.

Change-Id: I3741ab362bce14315f712ec24064ccc17e3578d4