Restrict the use of the C++17 attribute to C++17 (at least as best we

can given the current __cplusplus definitions).

Without this, Clang triggers TONS of warnings about using a C++17
extension. I tried using LLVM_EXTENSION to turn these off and it doesn't
work.

Suggestions on a better approach are welcome, but at least this makes
the build usable for me again.

llvm-svn: 278909