[clang-tidy] Fix mpi checks when running multiple TUs per clang-tidy process

Both the mpi-type-mismatch and mpi-buffer-deref check make use of a static MPIFunctionClassifier object.
This causes issue as the classifier is initialized with the first ASTContext that produces a match.
If the check is enabled on multiple translation units in a single clang-tidy process, this classifier won't be reinitialized for each TU. I'm not an expert in the MPIFunctionClassifier but I'd imagine this is a source of UB.
It is suspected that this bug may result in the crash caused here: https://bugs.llvm.org/show_bug.cgi?id=48985. However even if not the case, this should still be addressed.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D98275