[llvm-branch-commits] [lit] Expand late substitutions before running builtins (PR #165140)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/165140

This enables the use of readfile substitutions for populating
environment variables. This is necessary in some compiler-rt tests.