[Orc] Refactor the compile-on-demand layer to make module partitioning lazy,

[Orc] Refactor the compile-on-demand layer to make module partitioning lazy,
and avoid cloning unused decls into every partition.

Module partitioning showed up as a source of significant overhead when I
profiled some trivial test cases. Avoiding the overhead of partitionging
for uncalled functions helps to mitigate this.

This change also means that it is no longer necessary to have a
LazyEmittingLayer underneath the CompileOnDemand layer, since the
CompileOnDemandLayer will not extract or emit function bodies until they are
called.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236465 91177308-0d34-0410-b5e6-96231b3b80d8