build: keep header dependencies through a compiler cache hit on clang-cl (#15875)

For clang-cl, CMake sets the depfile flags to the gcc-style -MD, -MT and -MF,
passed through as -clang: arguments. ccache does not parse those, so a cache
hit writes only the object. Ninja has no depfile to read, so it records zero
header dependencies for that object and does not rebuild it after a header
edit. The stale object is still linked into the library and the DLL. sccache
0.15.0 reproduces the depfile on a hit and is unaffected.

Use /showIncludes instead. CMake already does that for MSVC, and ccache
reproduces it on a hit. A make-rules override sets the flags, because CMake
includes the override after Platform/Windows-Clang.cmake. It is forwarded to
each dependency because every one configures as its own CMake project, and it
is only set when the file exists, because scripts/flatpak/make_deps_tar.sh
packs deps/ alone.

In a build with a warm cache, 678 of 790 objects had no recorded headers.
Object code does not change. One GUI translation unit compiled both ways is
byte-identical apart from the COFF timestamp.

CMakeLists.txt

@@ -70,6 +70,9 @@ if (POLICY CMP0092)
     cmake_policy(SET CMP0092 NEW)
 endif ()
 
+# project() reads this, so set it first.
+set(CMAKE_USER_MAKE_RULES_OVERRIDE "${CMAKE_CURRENT_LIST_DIR}/cmake/modules/ClangClShowIncludes.cmake")
+
 project(OrcaSlicer)
 
 # Backward compatibility for old CMake versions