[clang] [clang-repl] Set up native platform support if ORC Runtime available (PR #217988)

[via cfe-commits]

khanxmetu wrote:

> Can we add a test case?

I added a unittest using the -debug output.  
Asserting the `LLJIT::getPlatformSupport()` type directly seemed difficult without RTTI

@vgvassilev 

https://github.com/llvm/llvm-project/pull/217988