[OpenMP][AMDGPU] Refactor setting uniform work group size attribute

Work group size attribute was set in Clang specific class. That's why
we cannot reuse this code in Flang.

If we move setting of this attribute to OpenMPIRBuilder, then we can reuse this
code in Flang and Clang. Function createOffloadEntry from OpenMPIRBuilder is
already used by Clang (via OpenMPIRBuilder::createOffloadEntriesAndInfoMetadata
function).

Differential Revision: https://reviews.llvm.org/D148525

Reviewed By: jdoerfert