[llvm] [BPF] Visit nested map array during BTF generation (PR #150608)

[via llvm-commits]

eddyz87 wrote:

First, I think that there is no need to modify `visitTypeEntry` at all, the changes can be confined to `visitMapDefType`, as in [this commit](https://github.com/llvm/llvm-project/commit/87326998aecd464723822a7429b6aaa9ff708be5).

Second, I think that `visitMapDefType` should be refactored to make intent more clear, as in [this commit](https://github.com/llvm/llvm-project/commit/86838de43e58ba733e6e691f3859da293c555949).

In both cases llvm unit tests require adjustments, kernel BPF selftests are passing, no significant changes in BPF generated for kernel BPF selftests.

https://github.com/llvm/llvm-project/pull/150608