[clang-repl] Factor out CreateJITBuilder() and allow specialization in derived classes (#84461)

The LLJITBuilder interface provides a very convenient way to configure
the ORCv2 JIT engine. IncrementalExecutor already used it internally to
construct the JIT, but didn't provide external access. This patch lifts
control of the creation process to the Interpreter and allows injection
of a custom instance through the extended interface. The Interpreter's
default behavior remains unchanged and the IncrementalExecutor remains
an implementation detail.