[clang] [Clang] Enable -fextend-lifetimes at -Og (PR #118026)

[Stephen Tozer via cfe-commits]

SLTozer wrote:

With the prior patches having landed, this is now in a state where it could land; I've rebased it onto main, meaning all of the code diff in this PR is now relevant.

https://github.com/llvm/llvm-project/pull/118026