[llvm] [Support] Fix buffer overflow in regcomp (PR #76681)

[David Blaikie via llvm-commits]

dwblaikie wrote:

No worries about the formatting - sticking with the formatting of the rest of the file is fine.

Does this need test coverage?

Is there any chance this bug is fixed in the upstream/original regex library this code is based on? Be nice to be able to just take their fix rather than coming up with our own if possible.

https://github.com/llvm/llvm-project/pull/76681