[OPENMP 4.5] Initial codegen for 'taskloop simd' directive.

OpenMP 4.5 defines 'taskloop simd' directive, which is combined
directive for 'taskloop' and 'simd' directives. Patch adds initial
codegen support for this directive and its 2 basic clauses 'safelen' and
'simdlen'.

llvm-svn: 267872