[CI] Add `-Wconversion` flag when building compiler in CI

[uditagarwal97]

Describe the bug

For context: #18786

To reproduce

Environment

Additional context

No response

[AlexeySachkov]

We kind of have it already in

llvm/llvm/cmake/modules/AddSecurityFlags.cmake

Line 71 in 6bcb265

add_compile_option_ext("-Wconversion" WCONVERSION)

However, that applies it globally and we can't realistically expect LLVM to build with this flag. Therefore, as part of this task, I suggest that we drop -Wconversion from AddSecurityFlags.cmake and apply it to specific sub-projects that we own ourselves.

[KornevNikita]

@uditagarwal97 I guess it's easy to find the right place, but just in case it probably should be here - https://github.com/intel/llvm/blob/sycl/sycl/CMakeLists.txt#L107

Also, it would be nice to have "-Wimplicit-fallthrough" for release builds, this flag is also a part of requirements (but only for the release type).