[ORC] Introduce SetUpExecutorNativePlatform utility.

Simplifies the process of building an LLJIT instance that supports the native
platform features (initializers, TLV, etc.).

SetUpExecutorNativePlatform can be passed to LLJITBuilder::setPlatformSetUp
method. It takes a reference to the ORC runtime (as a path or an in-memory
archive) and automatically sets the platform for LLJIT's ExecutionSession based
on the executor process's triple.

Differential Revision: https://reviews.llvm.org/D144276