[OMPIRBuilder] Add support for simdlen clause

[OMPIRBuilder] Add support for simdlen clause

This patch adds OMPIRBuilder support for the simdlen clause for the
simd directive. It uses the simdlen support in OpenMPIRBuilder when
it is enabled in Clang. Simdlen is lowered by OpenMPIRBuilder by
generating the loop.vectorize.width metadata.

Reviewed By: jdoerfert, Meinersbur

Differential Revision: https://reviews.llvm.org/D129149