Revert r344529 "Implement the first part of the calendar support for C++20"

Revert r344535 "Wrap up the new chrono literals in an #ifdef..."
Revert r344546 "Mark a couple of test cases as 'C++17-only'..."

Some of the buildbot failures were masked by another error,
and this one was probably missed.

llvm-svn: 344580