[llvm][Support] Make createUniquePath require at least one wildcard marker (#184688)
The API was a bit easy to accidentally mis-use, as it silently accepted
Model strings without any randomness marker characters (`%`), which
results in a unique path that's not at all unique. To avoid this
foot-gun, we now assert that the model contains at least one, and update
all the broken usages accordingly.
rdar://170349565