Allow creating standalone Regions

Currently, regions can only be constructed by passing in a `Function` or an
`Instruction` pointer referencing the parent object, unlike `Function`s or
`Instruction`s themselves that can be created without a parent.  It leads to a
rather complex flow in operation construction where one has to create the
operation first before being able to work with its regions.  It may be
necessary to work with the regions before the operation is created.  In
particular, in `build` and `parse` functions that are executed _before_ the
operation is created in cases where boilerplate region manipulation is required
(for example, inserting the hypothetical default terminator in affine regions).
Allow creating standalone regions.  Such regions are meant to own a list of
blocks and transfer them to other regions on demand.

Each instruction stores a fixed number of regions as trailing objects and has
ownership of them.  This decreases the size of the Instruction object for the
common case of instructions without regions.  Keep this behavior intact.  To
allow some flexibility in construction, make OperationState store an owning
vector of regions.  When the Builder creates an Instruction from
OperationState, the bodies of the regions are transferred into the
instruction-owned regions to minimize copying.  Thus, it becomes possible to
fill standalone regions with blocks and move them to an operation when it is
constructed, or move blocks from a region to an operation region, e.g., for
inlining.

PiperOrigin-RevId: 240368183