[llvm-jitlink] Add Process and Platform JITDylibs, generalize alias option.

The Process JITDylib holds reflected process symbols. The Platform JITDylib
holds ORC runtime symbols if the ORC runtime is loaded. The Platform and
Process JITDylibs are appended to the link order of all other JITDylibs,
including the main JITDylib, after any explicitly specified libraries. This
scheme is similar to the one introduced in LLJIT in 371cb1af, and makes
it easier to introduce aliases for process and platform symbols in a way that
affects all JITDylibs uniformly.

Since the Process and Platform JITDylibs are created implicitly the -alias
option is generalized to allow source and destination JITDylibs to be explicitly
specified, i.e. the -alias option now supports general re-exports.

Testcases are updated to account for the change.