[clang] Consistently use isOSSolaris()

While looking over the Solaris GNU ld patch (D85309
<https://reviews.llvm.org/D85309>), I noticed that we weren't using
`isOSSolaris()` consistenly in `clang`.  This patch fixes this.

Tested on `amd64-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D159222