[Orc] Add NotifyCreated callback for LLJITBuilder

[weliveindetail]

This is useful to attach generators to JITDylibs or inject initial symbol definitions.

[weliveindetail]

It seems like there are no tests for LLJITBuilder so far.

[weliveindetail]

Please note the differences to the existing callbacks SetupProcessSymbolsJITDylib and SetUpPlatform. They receive a reference to the LLJIT instance as well, but they both:

• are called during construction, i.e. before the last point of failure
• have a well-defined purpose that we would have to mimic
• have a default definition, that is inaccessible via the interface (e.g. setUpGenericLLVMIRPlatform)

[lhames]

I think it makes sense for NotifyCreated to return Error rather than void so that the callback itself can fail. Otherwise this looks great -- thanks @weliveindetail!

[joker-eph]

I don't know why the premerge CI didn't fail, but everywhere else in this function is return std::move(Err);

I made the change in 5d33f71

[joker-eph]

Seems like clang requires it but not gcc (which is used in premerge) nor MSVC.

You should have got a lot of buildbot emails though, didn't you?

[weliveindetail]

I got a lot of buildbot emails, but they also came all day for all kinds of unrelated reasons... Anyway, sorry for the mess.