Move non-common files from FortranCommon to FortranSupport (analogous to
LLVMSupport) such that

* declarations and definitions that are only used by the Flang compiler,
but not by the runtime, are moved to FortranSupport

* declarations and definitions that are used by both ("common"), the
compiler and the runtime, remain in FortranCommon

* generic STL-like/ADT/utility classes and algorithms remain in
FortranCommon

This allows a for cleaner separation between compiler and runtime
components, which are compiled differently. For instance, runtime
sources must not use STL's `<optional>` which causes problems with CUDA
support. Instead, the surrogate header `flang/Common/optional.h` must be
used. This PR fixes this for `fast-int-sel.h`.

Declarations in include/Runtime are also used by both, but are
header-only. `ISO_Fortran_binding_wrapper.h`, a header used by compiler
and runtime, is also moved into FortranCommon.