[lit] Add readfile substitution
This patch adds a new %{readfile:<file name>} substitution to lit. This
is needed for porting a couple of tests to lit's internal shell. These
tests are all using subshells to pass some option to a command are not
feasible to run within the internal shell without this functionality.
Reviewers: petrhosek, jh7370, ilovepi, cmtice
Reviewed By: jh7370, cmtice
Pull Request: https://github.com/llvm/llvm-project/pull/158441
