[Bug 39548][Clang] PGO bootstrap fails with python3: errors in perf-helper.py

Current clang fail to bootstrap in PGO mode when only python3 is available,
because perf-helper.py is not compatible with python3.

Commited on behalf of  Romain Geissler.

Differential Revision: https://reviews.llvm.org/D54071

llvm-svn: 350955