We should use `orc_rt` as the public C++ API namespace for the ORC runtime and
control symbol visibility to hide implementation details, rather than rely on
the '__' prefix.
Add unit test infrastructure for the ORC runtime, plus a cut-down
extensible_rtti system and extensible_rtti unit test.
Removes the placeholder.cpp source file.
Differential Revision: https://reviews.llvm.org/D102080