Add support to constant dense vector/tensor attribute.

The syntax of dense vecor/tensor attribute value is

`dense<` (tensor-type | vector-type)`,` attribute-list`>`

and

attribute-list ::= `[` attribute-list (`, ` attribute-list)* `]`.

The construction of the dense vector/tensor attribute takes a vector/tensor
type and a character array as arguments. The size of the input array should be
larger than the size specified by the type argument. It also assumes the
elements of the vector or tensor have been trunked to the data type sizes in
the input character array, so it extends the trunked data to 64 bits when it is
retrieved.

PiperOrigin-RevId: 217762811