[clang-tidy] Mock system headers for portability-restrict-system-includes tests.

Summary: Didn't realize that headers such as stddef.h may not exist on all systems. This patch mocks the headers so that the check's tests work on all systems.  (:

Reviewers: RKSimon, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: xazax.hun, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D76015