[clang] [HLSL] Don't invoke `dxv` from `clang-dxc` for text output (PR #135876)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

These tests that run `env PATH="" %clang_dxc ...` are problematic for my setup for running tests on Windows.

In my builds, I'm building with a dynamically linked `libc++.dll` provided by my toolchain, which is available in `$PATH`, so the built `bin/clang.exe` requires finding this `libc++.dll` when executed. Normally this works fine, but when invoking tests that do `env PATH="" %clang ...` then `%clang` will run in an environment where it no longer finds its required `libc++.dll` in `$PATH`, and thus fails.

Previously, we've waived such issues by omitting that kind of tests on Windows (where setting `PATH` also affects where dependent libraries are found) by wrapping `%if !system-windows %{ ... %}` around those bits - see f5a93c5f2a4d0916c975bbf028768d58a29b6b73.

Would you find it ok to do the same here? As these tests relate to HLSL, they're a bit more relevant for running on Windows, but tests clearing `PATH` are problematic for builds with a dynamically linked dependency, unfortunately.

https://github.com/llvm/llvm-project/pull/135876