[OPENMP] Codegen for 'private' clause in 'for' directive.

This patch generates helper variables which used as a private copies of the corresponding original variables inside an OpenMP 'for' directive. These generated variables are initialized by default (with the default constructor, if any). In OpenMP region references to original variables are replaced by the references to these private helper variables.
Differential Revision: http://reviews.llvm.org/D9106

llvm-svn: 235503