[libclc] Track dependencies through dependency files (#86965)

This commit fixes the problem of missing build dependencies between
libclc source files and their various includes (namely headers and .inc
files).

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. It should work
with all supported compilers and CMake generators.