In command lines with /showIncludes, clangd would output includes to stdout, breaking clients.
It's useful to have this test to verify the behavior at the clangd level, but the arg adjuster is a separate library and also used in other contexts.
The unit-test for this arg-adjuster lives in clang/unittests/Tooling/ToolingTest.cpp around line 500. Could you add a test for the new behavior there, too?
clang-cl suports both /foo and -foo style flags. See the top of clang/include/clang/Driver/CLCompatOptions.td
You probably want to add the -showIncludes prefix here too. Ideally this probably wouldn't do its own commandline parsing here (?)