[clang] [clang][Lex] Collapse relative extern module paths when recursing to prevent unbounded path length growth. (PR #193691)

[Tom Murray via cfe-commits]

TomMurray wrote:

> Any chance you could come up with a test for this change? Thanks (It seems reasonable otherwise)

Thanks for taking a look! You're right, I had a go at coming up with a test. Slightly tricky as the original failure case is kinda system dependent (ENAMETOOLONG), but I've tried to cover the unbounded path growth by just checking we're using normalised paths when parsing module files.

https://github.com/llvm/llvm-project/pull/193691