[OpenMP] Refactor the `target` function

[tianshilei1992]

Summary

Refactor the target function to make preparation of fixing the issue
of ahead-of-time device memory deallocation. This patch has following changes:

1. Replaced all mutex lock/unlock with std::lock_guard;
2. Renamed all variables to conform with LLVM code standard;
3. Took data process out of the target function.

[ye-luo]

Needs to split this patch into three.

1. function renaming. In addtion, should we update target_data_update as well?
2. std::lock_guard change.
3. "target" change.

The order of 1 and 2 can be flexible

[tianshilei1992]

I don't think it deserves three patches. The goal is to refactor the target function, and this patch just did this only thing. According to the bi-weekly meeting, the renaming could be with other related changes.

In D84778#2179502, @ye-luo wrote:

In addtion, should we update target_data_update as well?

I didn't touch target_data_update. Basically I only take care of related code.

[ye-luo]

In D84778#2179627, @tianshilei1992 wrote:

I don't think it deserves three patches. The goal is to refactor the target function, and this patch just did this only thing. According to the bi-weekly meeting, the renaming could be with other related changes.

In D84778#2179502, @ye-luo wrote:

In addtion, should we update target_data_update as well?

I didn't touch target_data_update. Basically I only take care of related code.

I think if massive renaming files and variables significantly introduce challenges for code review on the relevant change. It is more appropriate to do the renaming separately.

[tianshilei1992]

Will resubmit in three patches.