That's enabling Clang's -gkey-instructions, cc1's -gkey-instructions
remains off by default.

Key Instructions improves the optimized-code debug-stepping experience
in debuggers that use DWARF's `is_stmt` line table register to determine
stepping behaviour.

The feature can be disabled with -gno-key-instructions (note that the
positive and negative flag both imply -g).

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

Make the option visible, improve the help text, and add a release note.

Now PR 144104 has landed the flag is true by default (each DISubprogram tracks
whether or not it's using key instructions).

This needs to be driver level to pass an -mllvm flag to LLVM, though this may
change soon as the -mllvm flag will soon not be necessary.

Keep the flag help-hidden as the feature is under development.

This patch is part of a stack that teaches Clang to generate Key Instructions
metadata for C and C++.

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

The feature is only functional in LLVM if LLVM is built with CMake flag
LLVM_EXPERIMENTAL_KEY_INSTRUCTIONs. Eventually that flag will be removed.