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

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

There will be build failures because I haven't changed any tests.

In commit 3f96fe6d44774c8f892ff9b678c9cfa1d6df1539, Alexey added support for `reduction` on `target`, seemingly in response to a change in OpenMP 5.0.  The issue is that the 5.0 spec added support for `in_reduction`, not `reduction` to `target`.  The `reduction` clause is not allowed to this day.

Is this some kind of an extension we decided to implement?

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