[SYCL][libclc] Fix missing build dependencies (#13145)

This commit fixes the problem of missing build dependencies between
libclc source files and their various includes.

We would like to do this with compiler-generated dependency files
because then the dependencies are accurate and there are no false
positives, leading to unnecessary rebuilds. This is how regular C/C++
dependencies are usually tracked by CMake.

Note that this variable is an internal API so is not guaranteed to work,
but then again *all* of CMake's support for new languages (which we use
for CLC/LL languages) is an internal API. On balance this change is
probably worth it due to how minimally invasive it is.

The alternative would be to either:

1. list/glob all possible files in the directory as dependencies, which
would lead to false positives.
2. rewrite the library generation as a loop over all files and calling
`add_custom_command`, which can produce a dependency file (by tweaking
our clang command line) that can also be fed back to the same command
via the `DEPFILE` argument. This would be a much larger change and is
not as "neat".

libclc/cmake/CMakeCLCInformation.cmake

@@ -9,3 +9,4 @@ if(NOT CMAKE_CLC_CREATE_STATIC_LIBRARY)
 endif()
 
 set(CMAKE_INCLUDE_FLAG_CLC "-I")
+set(CMAKE_DEPFILE_FLAGS_CLC "-MD -MT <DEP_TARGET> -MF <DEP_FILE>")