* DescribeIEEESignaledExceptions() is unused on the device - warning.
* StopStatementText() could return while marked noreturn - warning.
* Including cuda/std/complex only in the device compilation
may cause nvcc to try to register variables in `cuda` namespace,
while they are not defined in the host compilation - error.
I decided to include cuda/std/complex always under RT_USE_LIBCUDACXX.
Mostly mechanical changes in preparation of extracting the Flang-RT
"subproject" in #110217. This PR intends to only move pre-existing files
to the new folder structure, with no behavioral change. Common files
(headers, testing, cmake) shared by Flang-RT and Flang remain in
`flang/`.
Some cosmetic changes and files paths were necessary:
* Relative paths to the new path for the source files and
`add_subdirectory`.
* Add the new location's include directory to `include_directories`
* The unittest/Evaluate directory has unitests for flang-rt and Flang. A
new `CMakeLists.txt` was introduced for the flang-rt tests.
* Change the `#include` paths relative to the include directive
* clang-format on the `#include` directives
* Since the paths are part if the copyright header and include guards, a
script was used to canonicalize those
* `test/Runtime` and runtime tests in `test/Driver` are moved, but the
lit.cfg.py mechanism to execute the will only be added in #110217.
