This removes ThreadSafeContext::Lock, ThreadSafeContext::getLock, and
ThreadSafeContext::getContext, and replaces them with a
ThreadSafeContext::withContextDo method (and const override).
The new method can be used to access an existing
ThreadSafeContext-wrapped LLVMContext in a safe way:
ThreadSafeContext TSCtx = ... ;
TSCtx.withContextDo([](LLVMContext *Ctx) {
// this closure has exclusive access to Ctx.
});
The new API enforces correct locking, whereas the old APIs relied on
manual locking (which almost no in-tree code preformed, relying instead
on incidental exclusive access to the ThreadSafeContext).
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