- Add support for fused locations.

These are locations that form a collection of other source locations with an optional metadata attribute.

- Add initial support for print/dump for locations.
Location Printing Examples:
* Unknown        : [unknown-location]
* FileLineColLoc : third_party/llvm/llvm/projects/google-mlir/test/TensorFlowLite/legalize.mlir:6:8
* FusedLoc       : <"tfl-legalize">[third_party/llvm/llvm/projects/google-mlir/test/TensorFlowLite/legalize.mlir:6:8, third_party/llvm/llvm/projects/google-mlir/test/TensorFlowLite/legalize.mlir:7:8]

- Add diagnostic support for fused locs.
* Prints the first location as the main location and the remaining as "fused from here" notes:
e.g.
third_party/llvm/llvm/projects/google-mlir/test/TensorFlowLite/legalize.mlir:6:8: error: This is an error.
  %1 = "tf.add"(%arg0, %0) : (i32, i32) -> i32
       ^
third_party/llvm/llvm/projects/google-mlir/test/TensorFlowLite/legalize.mlir:7:8: error: Fused from here.
  %2 = "tf.relu"(%1) : (i32) -> i32
       ^

PiperOrigin-RevId: 220835552