[clang] [llvm] [clang][llvm][aarch64][win] Add a clang flag and module attribute for import call optimization, and remove LLVM flag (PR #122831)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

LGTM with one minor comment.

https://github.com/llvm/llvm-project/pull/122831