[OPENMP] Allow to use global variables as lcv in loop-based directives.

For proper codegen we need to capture variable in the OpenMP region. In loop-based directives loop control variables are private by default and they must be captured in this region. There was a problem with capturing of globals, used as lcv, as they was not marked as private by default.
Differential Revision: http://reviews.llvm.org/D9336

llvm-svn: 236201