[ORC] Remove Layer handles from the layer concept.

[ORC] Remove Layer handles from the layer concept.

Handles were returned by addModule and used as keys for removeModule,
findSymbolIn, and emitAndFinalize. Their job is now subsumed by VModuleKeys,
which simplify resource management by providing a consistent handle across all
layers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324700 91177308-0d34-0410-b5e6-96231b3b80d8