[llvm-branch-commits] [libcxx] release/18.x: [libc++][modules] Re-add build dir CMakeLists.txt. (#81370) (PR #81651)

[Louis Dionne via llvm-branch-commits]

ldionne wrote:

Approved after talking with Mark. This is important for early adopters of modules.

https://github.com/llvm/llvm-project/pull/81651