[clang] [Clang] Add fake use emission to Clang with -fextend-lifetimes (PR #110102)

[Stephen Tozer via cfe-commits]

SLTozer wrote:

Post-holiday ping; I think all the outstanding comments have been addressed, with some minor changes since the last round of review - primarily referencing the new CodeGenOpts settings, and removing the O0-related logic.

https://github.com/llvm/llvm-project/pull/110102