Enable libc++ hardening.

[grynspan]

This PR enables libc++ hardening to help us catch C++ issues. (We don't know of any such issues, but we would have fixed them if we did!)

For more information, see https://libcxx.llvm.org/Hardening.html.

Checklist:

• [x] Code and documentation should follow the style of the Style Guide.
• [x] If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@jeffdav @compnerd @etcwilde Would you mind reviewing the CMake change in particular?

[grynspan]

@swift-ci please test

[grynspan]

@var-const Of interest to you too, I think?

[grynspan]

At desk, this makes CMake angry, so holding as a draft for now.