[mlir][linalg] Adapt FillOp to use a scalar operand.

Adapt the FillOp definition to use a scalar operand instead of a capture. This patch is a follow up to https://reviews.llvm.org/D104109. As the input operands are in front of the output operands the patch changes the internal operand order of the FillOp. The pretty printed version of the operation remains unchanged though. The patch also adapts the linalg to standard lowering to ensure the c signature of the FillOp remains unchanged as well.

Differential Revision: https://reviews.llvm.org/D104121