[llvm] [lit] Add LIT_CURRENT_TESTCASE environment variable when running tests (PR #168762)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> No idea if this would work, but could you do env > %t.txt followed by FileCheck %s --input-file %t.txt on the next line or something like that?

I still don't think that works. The issue is that env is executed as a builtin and doesn't have support for writing to the `stdin` file descriptor of whatever comes after it.

https://github.com/llvm/llvm-project/pull/168762