[llvm] [llvm][Support] Add support for executing a detached process (PR #81708)

[Connor Sughrue via llvm-commits]

cpsughrue wrote:

> Would you mind please explaining what is the purpose of this change? (in the description ideally) ~I know it's for the LLVM CAS but I'm not sure I understand exactly what is it for.~

I will add this to the description but I am working on a daemon that will make building modules easier and more efficient (https://discourse.llvm.org/t/rfc-modules-build-daemon-build-system-agnostic-support-for-explicitly-built-modules/71524). The module build daemon needs to be executed in a detached state so that it continues to run even when the clang invocation that spawned it terminates. Here is the first module build daemon pull request for context (#67562).

https://github.com/llvm/llvm-project/pull/81708