[OPENMP 4.0] Support for 'linear' clause in 'declare simd' directive.

[OPENMP 4.0] Support for 'linear' clause in 'declare simd' directive.

The linear clause declares one or more list items to be private to a SIMD lane and to have a linear relationship with respect to the iteration space of a loop.
'linear' '(' <linear-list> [ ':' <linear-step> ] ')'
When a linear-step expression is specified in a linear clause it must be
either a constant integer expression or an integer-typed parameter that is specified in a uniform clause on the directive.
The special this pointer can be used as if was one of the arguments to the function in any of the linear, aligned, or uniform clauses.

llvm-svn: 266056