This is actually not about a source repository being larger than a gigabyte, but a problem that arises with the C-preprocessor, where for a certain use, a certain header file is included many, many times, and where the position for all includes is kept. Typically a case of severe C-preprocessor abuse, I would say, which probably could be solved in a much better way. This shows how the C-preprocessor is both a blessing and a curse: it fixes many problems, but often causes compilation to be slow, because huge intermediate representations are produced which have to be parsed by the compiler.
Here's one, apparently: http://lists.llvm.org/pipermail/cfe-dev/2019-October/063459....