[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
19 files changed