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

[Alexandre Ganea via llvm-commits]

aganea wrote:

Looks good so far. Could you possibly add a unit test like https://github.com/llvm/llvm-project/blob/26d4afc3de86ca5416c8e38000362c526b6808cd/llvm/unittests/Support/ProgramTest.cpp#L287 please?

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