compiletest should note when test output is normalized

compiletest has two basic kinds of normalization:

1. normalization it does itself, either built-in or through a normalize-stderr pattern. this is what the UI suite uses.
2. normalization it doesn't know about, e.g. in a run-make test

normalizing outputs can lead to some extremely confusing errors, such as https://github.com/rust-lang/rust/pull/121571#issuecomment-2129928216. to avoid misleading contributors, it should note when the output has been normalized.

the first part is not too hard; it can save list of lines that have been modified in each test and print something like "note: 32/32 lines that did not match were normalized before comparison; see build/test/ui/foo.out.orig for unnormalized version"
(note that the unnormalized file already exists today but almost nobody knows about it).

the second part is trickier; run-make tests often run sed before comparing, and this is opaque to compiletest. perhaps fixing this can be part of the ongoing work to switch run-make to rust? and, in the meantime, introduce a new SED script, based off cut-and-grep.sh, to prevent the problem from getting worse? cc @jieyouxu

@rustbot label A-contributor-roadblock A-diagnostics D-confusing A-testsuite

Start by reading the compiletest normalization flow and the existing unnormalized output at build/test/ui/foo.out.orig. Compare how built-in and normalize-stderr changes are tracked, then inspect cut-and-grep.sh and run-make handling. Done means normalized output is clearly reported with a path to the unnormalized version, without misleading comparison errors.