[OPENMP]Add support for analysis of linear variables and step.

Summary:
Added support for basic analysis of the linear variables and linear step
expression. Linear loop iteration variables must be excluded from this
analysis, only non-loop iteration variables must be analyzed.

Reviewers: NoQ

Subscribers: guansong, cfe-commits, caomhin, kkwli0

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65461

llvm-svn: 368295