[clang] [flang] Reland "[Flang][Driver] Add a flag to control zero initialization" (PR #123606)

[Tom Eccles via cfe-commits]

https://github.com/tblah approved this pull request.

LGTM. As I understand it the test failures were fixed by https://github.com/llvm/llvm-project/pull/123364

https://github.com/llvm/llvm-project/pull/123606