[OPENMP]Add support for analysis of firstprivate variables.
Firstprivate variables are the variables, for which the private copies
must be created in the OpenMP regions and must be initialized with the
original values. Thus, we must report if the uninitialized variable is
used as firstprivate.
Subscribers: guansong, jdoerfert, caomhin, kkwli0, cfe-commits
Differential Revision: https://reviews.llvm.org/D64765