[clang] [clang-scan-deps] Move command-line generation out of critical section (PR #158187)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 approved this pull request.

LGTM, good catch! The pointers in `NewMDs` should be stable, since they're stored in `std::unordered_map`, so doing this outside of the critical section should be fine, I think.

https://github.com/llvm/llvm-project/pull/158187