[llvm] [lit] Add readfile substitution (PR #158441)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Out of interest could you point me to these tests? Are they just doing something like echo $(cat /some/file) or something more complicated. Adding this substitution sounds good to me since it's much easier than supporting "`pwd`" or "$(pwd)".

They're usually a bit more complicated. There are a bit under 10 tests in clang that need this in #158446. There are maybe a dozen or so compiler-rt tests. For clang it's usually things like getting the default target triple or manipulating resource directories.

https://github.com/llvm/llvm-project/pull/158441