Revert "Revert "[Concepts] Add TODO and requires-clause placeholder; NFC""
Revert "Revert "[Concepts] Add TODO and requires-clause placeholder; NFC""

This reverts commit 5e281ba2bdb3c221b495a9e822d70e77d4bc06e6.

This restores r277495 now that the relevant LLVM/clang changes have
been brought into GitHub swift-llvm/stable-next and
swift-clang/stable-next.

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb