[clang] [test] Fix clang-cl unused argument tests on paths that start with /U

This reinstates a test that was temporarily removed in
e26bbae3, in a form that works on
Darwin.

Use -LD instead of -link as a linker argument that is unused when
compiling, that produces warnings normally. -LD can be placed anywhere
in the command line, so that the command line ends with "-- %s", making
paths starting with /U correctly interpreted as paths, not options.