[llvm] [OpenMP] Fix crash with duplicate mapping on target directive (PR #146136)

[Julian Brown via llvm-commits]

jtb20 wrote:

(Generally it'd be nice if the compiler unified these kinds of mapping before the runtime sees them, but the runtime should still be able to cope with the degenerate cases rather than crashing.)

https://github.com/llvm/llvm-project/pull/146136