[clang] [Driver] Fix _XOPEN_SOURCE definition on Solaris (PR #137141)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

(I have limited internet access between April 20th and May 4th, and my response time may be delayed..)


https://github.com/llvm/llvm-project/pull/137141