[OPNEMP]Allow grainsize clause in combined task-based directives.

The expression of the grainsize clause must be captured in the combined
task-based directives, like 'parallel master taskloop' directive.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374810 91177308-0d34-0410-b5e6-96231b3b80d8