Cache external CodeInstances (#43990)
Prior to this PR, Julia's precompiled `*.ji` files saved just two
categories of code: unspecialized method definitions and
type-specialized code for the methods defined by the package. Any
novel specializations of methods from Base or previously-loaded
packages were not saved, and therefore effectively thrown away.
This PR caches all the code---internal or external---called during
package definition that hadn't been previously inferred, as long
as there is a backedge linking it back to a method owned by
a module being precompiled. (The latter condition ensures it will
actually be called by package methods, and not merely transiently
generated for the purpose of, e.g., metaprogramming or variable
initialization.) This makes precompilation more intuitive (now it
saves all relevant inference results), and substantially reduces
latency for inference-bound packages.
Closes #42016
Fixes #35972
Issue #35972 arose because codegen got started without re-inferring
some discarded CodeInstances. This forced the compiler to insert a
`jl_invoke`. This PR fixes the issue because needed CodeInstances are
no longer discarded by precompilation.
