[MLIR] Fuse locations of merged constants (#74670)

When merging constants by the operation folder, the location of the op
that remains should be updated to track the new meaning of this op. This
way we do not lose track of all possible source locations that the
constant op came from, and the final location of the op is less reliant
on the order of folding. This will also help debuggers understand how to
step these instructions.

This PR introduces a helper for operation folder to fuse another
location into the location of an op. When an op is deduplicated, fuse
the location of the op to be removed into the op that is retained. The
retained op now represents both original ops.

The FusedLoc will have a string metadata to help understand the reason
for the location fusion (motivated by the
[example](https://github.com/llvm/llvm-project/blob/71be8f3c23497e28c86f1135f564b16106d8d6fb/mlir/include/mlir/IR/BuiltinLocationAttributes.td#L130)
in the docstring of FusedLoc).