[clang] Temporarily disable Darwin test for linking against libc++ on non-darwin systems (#170912)

Disable the test added in #170303, which breaks bots that don't use ld
as their linker. This is a temporary and narrow disablement of the test
until we can make it more general again, to get the bots green.

Co-authored-by: Louis Dionne <[email protected]>

Author: Kewen12

clang/test/Driver/darwin-link-libcxx.cpp

@@ -1,5 +1,8 @@
 // UNSUPPORTED: system-windows
 
+// TODO: Make this test portable across platforms
+// REQUIRES: system-darwin
+
 // Tests to check that we link against the toolchain-provided libc++ built library when it is provided.
 // This is required to prefer the toolchain's libc++ over the system's libc++, which matches the behavior
 // we have for header search paths.