For the record, I have changed my minimum not-working example linked in the LLVM Discourse thread to use a fully clang-only toolchain.
I changed line 14 of CMakePresets.json to clang.exe, but I still receive the same error. test.pcm is built in ${CMAKE_BINARY_DIR}/CMakeFiles/test_lib.dir/test.pcm. A full, verbose build log is below:
====================[ Build | test | Ninja - Ninja ]============================
cmake.exe --build --target test --preset Ninja
[1/8] cmd.exe /C ""C:/Program Files/LLVM/bin/clang-scan-deps.exe" -format=p1689 -- C:\PROGRA~1\LLVM\bin\clang.exe -O0 -std=gnu++20 -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -g -Xclang -gcodeview -x c++ D:/Libraries/Downloads/test/main.cpp -c -o CMakeFiles\test.dir\main.cpp.obj -MT CMakeFiles\test.dir\main.cpp.obj.ddi -MD -MF CMakeFiles\test.dir\main.cpp.obj.ddi.d > CMakeFiles\test.dir\main.cpp.obj.ddi"
[2/8] cmd.exe /C ""C:/Program Files/LLVM/bin/clang-scan-deps.exe" -format=p1689 -- C:\PROGRA~1\LLVM\bin\clang.exe -O0 -std=gnu++20 -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -g -Xclang -gcodeview -x c++ D:/Libraries/Downloads/test/test.cppm -c -o CMakeFiles\test_lib.dir\test.cppm.obj -MT CMakeFiles\test_lib.dir\test.cppm.obj.ddi -MD -MF CMakeFiles\test_lib.dir\test.cppm.obj.ddi.d > CMakeFiles\test_lib.dir\test.cppm.obj.ddi"
[3/8] "C:\Program Files\CMake\bin\cmake.exe" -E cmake_ninja_dyndep --tdi=CMakeFiles\test_lib.dir\CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=CMakeFiles/test_lib.dir/CXX.dd @CMakeFiles/test_lib.dir/CXX.dd.rsp
[4/8] "C:\Program Files\CMake\bin\cmake.exe" -E __run_co_compile --tidy=clang-tidy;-checks=-*,readability-*;--extra-arg=-Xclang=-fprebuilt-module-path=D:/Libraries/Downloads/test/build/CMakeFiles/test_lib.dir/;-p=D:/Libraries/Downloads/test/build;--extra-arg-before=--driver-mode=g++ --source=D:/Libraries/Downloads/test/test.cppm -- C:\PROGRA~1\LLVM\bin\clang.exe -O0 -std=gnu++20 -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -g -Xclang -gcodeview -MD -MT CMakeFiles/test_lib.dir/test.cppm.obj -MF CMakeFiles\test_lib.dir\test.cppm.obj.d @CMakeFiles\test_lib.dir\test.cppm.obj.modmap -o CMakeFiles/test_lib.dir/test.cppm.obj -c D:/Libraries/Downloads/test/test.cppm
D:\Libraries\Downloads\test\test.cppm:5:30: warning: 5 is a magic number; consider replacing it with a named constant [readability-magic-numbers]
auto f() -> int { return 1 + 5; }
^
[5/8] cmd.exe /C "cd . && "C:\Program Files\CMake\bin\cmake.exe" -E rm -f test_lib.lib && C:\PROGRA~1\LLVM\bin\llvm-ar.exe qc test_lib.lib CMakeFiles/test_lib.dir/test.cppm.obj && C:\PROGRA~1\LLVM\bin\LLVM-R~1.EXE test_lib.lib && cd ."
[6/8] "C:\Program Files\CMake\bin\cmake.exe" -E cmake_ninja_dyndep --tdi=CMakeFiles\test.dir\CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=CMakeFiles/test.dir/CXX.dd @CMakeFiles/test.dir/CXX.dd.rsp
[7/8] "C:\Program Files\CMake\bin\cmake.exe" -E __run_co_compile --tidy=clang-tidy;-checks=-*,readability-*;--extra-arg=-Xclang=-fprebuilt-module-path=D:/Libraries/Downloads/test/build/CMakeFiles/test_lib.dir/;-p=D:/Libraries/Downloads/test/build;--extra-arg-before=--driver-mode=g++ --source=D:/Libraries/Downloads/test/main.cpp -- C:\PROGRA~1\LLVM\bin\clang.exe -O0 -std=gnu++20 -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -g -Xclang -gcodeview -MD -MT CMakeFiles/test.dir/main.cpp.obj -MF CMakeFiles\test.dir\main.cpp.obj.d @CMakeFiles\test.dir\main.cpp.obj.modmap -o CMakeFiles/test.dir/main.cpp.obj -c D:/Libraries/Downloads/test/main.cpp
FAILED: CMakeFiles/test.dir/main.cpp.obj
"C:\Program Files\CMake\bin\cmake.exe" -E __run_co_compile --tidy=clang-tidy;-checks=-*,readability-*;--extra-arg=-Xclang=-fprebuilt-module-path=D:/Libraries/Downloads/test/build/CMakeFiles/test_lib.dir/;-p=D:/Libraries/Downloads/test/build;--extra-arg-before=--driver-mode=g++ --source=D:/Libraries/Downloads/test/main.cpp -- C:\PROGRA~1\LLVM\bin\clang.exe -O0 -std=gnu++20 -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -g -Xclang -gcodeview -MD -MT CMakeFiles/test.dir/main.cpp.obj -MF CMakeFiles\test.dir\main.cpp.obj.d @CMakeFiles\test.dir\main.cpp.obj.modmap -o CMakeFiles/test.dir/main.cpp.obj -c D:/Libraries/Downloads/test/main.cpp
D:/Libraries/Downloads/test/main.cpp:1:8: fatal error: module 'test' not found
import test;
~~~~~~~^~~~
1 error generated.
ninja: build stopped: subcommand failed.