MPI-Checker: move MPIFunctionClassifier.h

Summary:
This patch moves the MPIFunctionClassifier header to `clang/include/clang/StaticAnalyzer/Checkers`,
in order to make it accessible in other parts of the architecture.

Reviewers: dcoughlin, zaks.anna

Subscribers: alexfh, cfe-commits

Patch by Alexander Droste!

Differential Revision: https://reviews.llvm.org/D22671

llvm-svn: 276639