Implement P0548: 'common_type and duration' This involves a subtle change in the return type of the unary +/- operators for std::chrono::duration, though I expect that no one will notice.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@298416 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed