[flang-rt][openmp] Add file-level dependencies for builtin mod files (#204260)

CMake currently intentionally ignores intrinsic dependencies. flang-rt
already had a workaround using target-level dependencies, but it does
not know about dependencies between .mod files created within the same
add_library that CMake ignores. As a result, as reported in #203549,
updating a .mod did not trigger rebuilding the .mod files that depend on
it. Specifically, .mod files store the checksum of used .mod files which
need to be updated and therefore require transitive rebuidling.

As mentioned, CMake already adds this file-level dependency itself for
non-intrinsic modules dependencies. In this PR we are injecting the
additional dependencies that CMake does not add via OBJECT_DEPENDS.

Three caveats:

1. Using OBJECT_DEPENDS for dependencies between modules of the same
OBJECT library makes Ninja complain about circular dependencies. To
avoid, split __fortran_builtins.f90 and __cuda_builtins.f90 into their
own OBJECT libraries each.

2. The dependency cannot be on the .mod files. Because of how CMake's
dependency mechanism works, the dependency scanning runs only after
ensuring that the OBJECT_DEPENDS files exist. So if they don't exist
yet, we get a missing dependency error. To avoid, we depend on the .o
file instead.

3. Generator expressions do not work in OBJECT_DEPENDS. That is, we
cannot use `$<TARGET_FILES:flang_rt.mod.fortran.builtins>`, but
fortunately we can use OBJECT_OUTPUTS to make the location predictable.

At some point in the future,
https://gitlab.kitware.com/cmake/cmake/-/issues/26803 should be resolved
and become the minimum required version to build LLVM, in which case
these workarounds can be removed.

GitOrigin-RevId: a92039c3e44e102f90751c8e8b2ab70a077016b7