Modify the TypeCategory for quad-precision COMPLEX to
CFI_type_float128_Complex so it matches the TypeCode returned
by SELECT TYPE lowering.

Fixes #134565

When somebody replaced uses of std::optional<> in the runtime with a new
optional<> defined locally, many needless top-level Fortran:: namespace
qualifiers were added, which are inconsistent with namespace usage in
the runtime. Clean them up.

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.