[llvm] [Orc] Add NotifyCreated callback for LLJITBuilder (PR #84175)

[Stefan Gränitz via llvm-commits]

weliveindetail wrote:

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`)

https://github.com/llvm/llvm-project/pull/84175