[perf-training] Change profile file pattern string to use %4m instead of %p

Summary: With %p, each test file that we're using to generate profile data will make its own profraw file which is around 60 MB in size. If we have a lot of test files, that quickly uses a lot of space. Use %4m instead to share the profraw files used to store the profile data. We use 4 here based on the default value in https://reviews.llvm.org/source/llvm-github/browse/master/llvm/CMakeLists.txt$604

Reviewers: beanz, phosek, xiaobai, smeenai, vsk

Reviewed By: vsk

Subscribers: vsk, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71585