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

[Rainer Orth via cfe-commits]

rorth wrote:

This patch is required to build `flang` on Solaris/amd64.  Until very recently, a 2-stage build of `main` with `flang` included worked with `clang-20` as build compiler since `flang` was only built in stage 2 and the freshly built `clang-21` does include the patch..  However, now `flang` is built in stage 1 already, so the stage 1 compiler needs to include this patch, too, to avoid the build breaking again.

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