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

[Rainer Orth via cfe-commits]

rorth wrote:

Drats, I'd hoped the markup would inhibit the cherry pick directive from being picked up.

I've now dropped the patch into local LLVM 20.1.5 builds on both `amd64-pc-solaris2.11` and `sparcv9-sun-solaris2.11`.  It applied cleanly and introduced no regressions.  I'll try a regular cherry pick again: maybe it finally works this time.


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