[OPENMP 4.0] Parsing/sema analysis for 'simdlen' clause in 'declare simd'

construct.

OpenMP 4.0 defines '#pragma omp declare simd' construct that may have
associated 'simdlen' clause with constant positive expression as an
argument:
simdlen(<const_expr>)
Patch adds parsin and semantic analysis for simdlen clause.

llvm-svn: 265668