[ORC] Support non-moving cloneToContext

[jeaye]

There are two usability concerns, in some cases, with the new cloneToContext from #146852.

1. This API assumes the LLVM context is in a ThreadSafeContext, but I have no need for that. My IR gen is single-threaded. IR gen (via IRBuilder) uses the context and module often, so I would need to do a whole lot of withContextDo with closures just to do things like create a new basic block.

2. The new cloning function requires a ThreadSafeModule, which can only be constructed by moving (i.e. consuming) a module. This prevents me from caching my C++ modules, which explodes the number I need to generate. Cloning should be read-only, so the module consuming here feels like it's just an implication of TSMs being used.

We can address both of these by:

1. Adding a getContextUnlocked to ThreadSafeContext, following what exists for ThreadSafeModule; this allows me to store a raw pointer for easy use with IR gen

2. Add an overload for cloneToContext which take a const Module & and use that as the implementation for the TSM version

@lhames for vis.

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