[clang] [flang] [lld] [Flang] Optionally do not compile the runtime in-tree (PR #122336)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 approved this pull request.

Makes sense, should be fine to transition since we just hit a release milestone.

https://github.com/llvm/llvm-project/pull/122336