macOS Sequoia's linker will complain if RPATHs on the CLI are specified more than once.
To avoid errors due to this, make `cc` only append unique RPATHs to the final args list.
This required a few improvements to the logic in `cc`:
1. List functions in `cc` didn't have any way to append unique elements to a list. Add a
`contains()` shell function that works like our other list functions. Use it to implement
an optional `"unique"` argument to `append()` and an `extend_unique()`. Use that to add
RPATHs to the `args_list`.
2. In the pure `ld` case, we weren't actually parsing `RPATH` arguments separately as we
do for `ccld`. Fix this by adding *another* nested case statement for raw `RPATH`
parsing. There are now 3 places where we deal with `-rpath` and friends, but I don't
see a great way to unify them, as `-Wl,`, `-Xlinker`, and raw `-rpath` arguments are
all ever so slightly different.
3. Fix ordering of assertions to make `pytest` diffs more intelligible. The meaning of
`+` and `-` in diffs changed in `pytest` 6.0 and the "preferred" order for assertions
became `assert actual == expected` instead of the other way around.
Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
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