This is the next step in the line of work started by #33955,
though a lot of enabling work towards this was previously done
by Jameson in his codegen-norecursion branch. The basic
thrust here is to allow external packages to manage their own
cache of compiled code that may have been generated using entirely
difference inference or compiler options. The GPU compilers are one
such example, but there are several others, including generating
code using offload compilers, such as XLA or compilers for secure
computation. A lot of this is just moving code arround to make it
clear exactly which parts of the code are accessing the internal
code cache (which is now its own type to make it obvious when
it's being accessed), as well as providing clear extension points
for custom cache implementations. The second part is to refactor
CodeInstance construction to separate construction and insertion
into the internal cache (so it can be inserted into an external
cache instead if desired). The last part of the change
is to give cgparams another hook that lets the caller replace
the cache lookup to be used by codegen.

Co-authored-by: Tim Besard <[email protected]>

Co-authored-by: Jameson Nash <[email protected]>