[OPENMP 4.1] Initial support for modifiers in 'linear' clause.

[OPENMP 4.1] Initial support for modifiers in 'linear' clause.

OpenMP 4.1 adds 3 optional modifiers to 'linear' clause.
Format of 'linear' clause has changed to:
```
linear(linear-list[ : linear-step])
```
where linear-list is one of the following
```
list
modifier(list)
```
where modifier is one of the following:
```
 ref (C++)
 val (C/C++)
 uval (C++)
```
Patch adds parsing and sema analysis for these modifiers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245550 91177308-0d34-0410-b5e6-96231b3b80d8