Summary:
Before this change, the default backend was TBB but one could disable
anything related to TBB by removing the parallel policies. This change
uses the serial backend by default and removes the ability to disable
parallel policies, which is not useful anymore.
Reviewers: rodgert, MikeDvorskiy
Subscribers: mgorny, jkorous, dexonsmith, jdoerfert, libcxx-commits
Differential Revision: https://reviews.llvm.org/D59792
llvm-svn: 359134
Louis Dionne