gdb/registry: add registry::key::try_emplace

gdb/registry: add registry::key::try_emplace

We have many times the pattern:

    current_source_location *loc
      = current_source_key.get (pspace);
    if (loc == nullptr)
      loc = current_source_key.emplace (pspace);
    return loc;

I thought it would be nice to have it directly part of registry::key.
Add a try_emplace method, which is like emplace, except that it returns
the existing data, if any.  The try_emplace name comes from similar
methods in std::map or gdb::unordered_map
(ankerl::unordered_dense::map).

Replace as many callers as I could find to use it.

Change-Id: I21f922ca065a354f041caaf70484d6cfbcbb76ed
Approved-By: Tom Tromey <tom@tromey.com>