[flang][mlir] Add checks and test for linear clause on omp.wsloop and omp.simd (#174916)
This PR adds additional checks and tests for linear clause on omp.wsloop
and omp.simd (both standalone and composite). For composite simd
constructs, the translation to LLVMIR uses the same
`LinearClauseProcessor` under `convertOmpSimd`, as already present in
previous PRs like https://github.com/llvm/llvm-project/pull/150386 and
https://github.com/llvm/llvm-project/pull/139386