[OpenMP][FIX] Enforce a function boundary for a new data environment

Whenever we enter a new OpenMP data environment we want to enter a
function to simplify reasoning. Later we probably want to remove the
entire specialization wrt. the if clause and pass the result to the
runtime, for now this should fix PR48686.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D94315