[llvm] [gn build] Allow specifying macOS deployment target (PR #107492)

[Daniel Bertalan via llvm-commits]

BertalanD wrote:

Theoretically, a lower bound for local development is 10.13, as that's the earliest version supported by Xcode 10, which is the minimum according to the docs.

DWARF 4 needs 10.11, chained fixups need 12.0, and 15.0 will move to DWARF 5.

What do you think, is 12.0 too new?

https://github.com/llvm/llvm-project/pull/107492