[perf-training] Ignore 'Profile Note' warnings from the runtime

After r272599, -DLLVM_BUILD_INSTRUMENTED passes a default argument to
-fprofile-instr-generate. This confuses the perf-helper script because
the runtime emits a note stating that the default is overridden by the
LLVM_PROFILE_FILE environment variable.

Change the perf-helper script s.t it does not treat these notes as
failures.

This isn't a strictly NFC change, but I don't see a simple way to add a
test for it.

llvm-svn: 272695