This is a generalization of the existing cloneToNewContext operation:
rather than cloning the given module into a new ThreadSafeContext it
clones it into any given ThreadSafeContext. The given ThreadSafeContext
is locked to ensure that the cloning operation is safe.

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).

R and K are already of Edge::Kind in all these cases.

ArrayRef now has a new constructor that takes a parameter whose type
has data() and size().  This patch migrates:

  ArrayRef<T>(X.data(), X.size()

to:

  ArrayRef<T>(X)

This commit adds operations to orc::MemoryAccess for reading basic types
(uint8_t, uint16_t, uint32_t, uint64_t, pointers, buffers, and strings)
from executor memory.

The InProcessMemoryAccess and EPCGenericMemoryAccess implementations are
updated to support the new operations.

This moves the MemoryAccess interface out of the ExecutorProcessControl
class and splits implementation classes InProcessMemoryManager and
SelfExecutorProcessControl out of ExecutorProcessControl.h and into
their own headers.

This include is only used for implementation, so shouldn't be in the public
header.

This method was included in the original Plugin API as a counterpart to
JITEventListener::notifyLoaded but was never used.

When JITLink reports an out-of-range error, the underlying reason could
be hidden from the user if it's due to an excessively large target
addend. Add non-zero target addend to the message for clarity.