[llvm] [Frontend][OpenMP] Remove `reduction` from allowed clauses for `target` (PR #90754)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

Is everybody ok with removing it, and removing the parts of the clang tests that use it?

If not, what else should we do?  Having it allowed in the .td file will affect flang too.  We could implement `isAllowedClauseForDirectiveEx` in clang that would keep reduction on target, and defer everything else to the existing `isAllowedClauseForDirective`.

https://github.com/llvm/llvm-project/pull/90754