[OPENMP]Add support for analysis of if clauses.

Summary:
Added support for analysis of if clauses in the OpenMP directives to be
able to check for the use of uninitialized variables.

Reviewers: NoQ

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64646

llvm-svn: 366068