Allow C-API users to debug their JITed code via the GDB JIT Interface.
This is currently supported on ELF and MachO based platforms. On
other systems `LLVMOrcLLJITEnableDebugSupport()` returns an error.
This patch adds a new C-API header `LLJITUtils.h`, which can host
further advanced JIT features in the future. Using the header requires
linking against LLVMOrcDebugging.
This re-applies db51e572893, which was reverted in 05b1a2cb3e6 due to bot
failures. The DebuggerSupportPlugin now depends on DWARF, so it has been moved
to the new OrcDebugging library (as has the enableDebuggerSupport API).
