[OPENMP]Add support for analysis of reduction variables.

Summary:
Reduction variables are the variables, for which the private copies
must be created in the OpenMP regions. Then they are initialized with
the predefined values depending on the reduction operation. After exit
from the OpenMP region the original variable is updated using the
reduction value and the value of the original reduction variable.

Reviewers: NoQ

Subscribers: guansong, jdoerfert, caomhin, kkwli0, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65106

llvm-svn: 367116