Using LIT in out-of-tree projects

Writing an in-tree LLVM IR regression test is pretty easy: all you need to do is annotate the IR file with testing directives. Look at the following script, for example:

; RUN: opt < %s -instcombine -S -o - | FileCheck %s
target triple = "x86_64-unknown-linux"
define i32 @foo(i32 %c) {
entry:
  ; CHECK: [[RET:%.+]] = add nsw i32 %c, 3
  ; CHECK: ret i32 [[RET]]
  %add1 = add nsw i32 %c, 1
  %add2 = add nsw i32 %add1, 2
  ret i32 %add2
}

This script checks if InstCombine (triggered by the -instcombine command-line option shown in the preceding snippet) simplifies two succeeding arithmetic adds into one. After putting this file into an arbitrary folder under llvm/test, the script will automatically be picked and run as part of the regression test when you're executing the llvm-lit command-line tool.

Despite its convenience, this barely helps you use LIT in out-of-tree projects...