[OpenMP][libomptarget] Implement memory lock/unlock API in NextGen plugins

This patch implements the memory lock/unlock API, introduced in patch https://reviews.llvm.org/D139208,
in the NextGen plugins. Locked buffers feature reference counting and we allow certain overlapping. Given
an already locked buffer A, other buffers that are fully contained inside A can be locked again, even if
they are smaller than A. In this case, the reference count of locked buffer A will be incremented. However,
extending an existing locked buffer is not allowed. The original buffer is actually unlocked once all its
users have released the locked buffer and sub-buffers (i.e., the reference counter becomes zero).

Differential Revision: https://reviews.llvm.org/D141227

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