[CMake] Unable to locate lib/DxcSupport/dxcapi.use.cpp when building from a parent project

[sergeiepopov]

Description
When I add dxc as a dependency to my project through add_subdirectory("external/dxc"), it now fails to build because this line is unable to correctly locate /lib/DxcSupport/dxcapi.use.cpp due to ${CMAKE_SOURCE_DIR} now pointing to my top-level project path rather than the dxc root path.
The regression was introduced in the #7514

Steps to Reproduce
Create a CMake project and add dxc as a dependency using add_subdirectory("external/dxc").

Actual Behavior

CMake Error at core/external/dxc/cmake/modules/AddLLVM.cmake:611 (add_executable):
Cannot find source file:

C:/source/engine/lib/DxcSupport/dxcapi.use.cpp

Environment

• DXC version latest master
• Host Operating System Windows 11

[sergeiepopov]

@bob80905 for viz.

[sergeiepopov]

The following CMake variables contain the path that points to the correct dxc directory, which should be used in place of ${CMAKE_SOURCE_DIR}:

PROJECT_SOURCE_DIR=C:/source/engine/core/external/dxc
LLVM_SOURCE_DIR=C:/source/engine/core/external/dxc
LLVM_MAIN_SRC_DIR=C:/source/engine/core/external/dxc

[toji]

We are encountering the same issue when attempting to roll this change into Dawn (Chrome's WebGPU implementation).