%t is currently documented as:
temporary file name unique to the test
https://llvm.org/docs/CommandGuide/lit.html#substitutions
Which I take to mean if the path is a/b/c/tempfile, then %t would be
tempfile. It is not, it's the whole path.
(which is hinted at by %basename_t, but why would you read that if you
didn't need to use it)
As seen in #164396 this can create confusion when people use it as if it
were just the file name.
Make it clear in the docs that this is a unique path, which can be used
to make files or folders.
ed87795aa6
