[Clang] Enable -fextend-lifetimes at -Og (#118026)
Recently, a new flag -fextend-variable-liveness was added that prevents
optimizations from removing the values of source variables in some cases,
improving the quality of debugging for optimized builds where it is
enabled.
Following the inclusion of the flag, this patch enables it by default
when `-Og` is set. Currently, `-Og` is equivalent to `-O1` - it is
effectively just an alias. By enabling `-fextend-lifetimes`, this patch
changes the code generated by Clang with `-Og` to have reduced
optimization and greater debuggability than `-O1`, differentiating the
two according to their respective purposes. This idea was discussed
previously on Discourse where there was general agreement with the
principle of this change:
https://discourse.llvm.org/t/rfc-redefine-og-o1-and-add-a-new-level-of-og
