[llvm] [Support][NFC] Add test documenting that empty `Regex` pattern matches nothing. (PR #83849)

[David Blaikie via llvm-commits]

dwblaikie wrote:

if this is different from other regex implementations, could we make it do something more like other implementations so as to be less surprising? (is this a feature of the underlying regex library we use - or something we added on top?)

Couldn't find clear documentation in the libc regex (at a glance/quick googling) that this is all based on - but doesn't look like we special case it in any way, so probably a feature of libc's regex... 

https://github.com/llvm/llvm-project/pull/83849