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

[via llvm-commits]

DavidKorczynski wrote:

> Does this need test coverage?

I'm not strongly opinionated on this: OSS-Fuzz will catch and flag the bug if a regression happens, but having a test will make it more obvious, and instantaneous, if a regression happens. Added a test.

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