Implement support for constant folding operations and a simple constant folding

optimization pass:

 - Give the ability for operations to implement a constantFold hook (a simple
   one for single-result ops as well as general support for multi-result ops).
 - Implement folding support for constant and addf.
 - Implement support in AbstractOperation and Operation to make this usable by
   clients.
 - Implement a very simple constant folding pass that does top down folding on
   CFG and ML functions, with a testcase that exercises all the above stuff.

Random cleanups:
 - Improve the build APIs for ConstantOp.
 - Stop passing "-o -" to mlir-opt in the testsuite, since that is the default.

PiperOrigin-RevId: 213749809