Densify storage for f16, f32 and support f16 semantics in FloatAttrs

Existing implementation always uses 64 bits to store floating point values in
DenseElementsAttr.  This was due to FloatAttrs always a `double` for storage
independently of the actual type.  Recent commits added support for FloatAttrs
with the proper f32 type and floating semantics and changed the bitwidth
reporting on FloatType.

Use the existing infrastructure for densely storing 16 and 32-bit values in
DenseElementsAttr storage to store f16 and f32 values.  Move floating semantics
definition to the FloatType level.  Properly support f16 / IEEEhalf semantics
at the FloatAttr level and in the builder.

Note that bf16 is still stored as a 64-bit value with IEEEdouble semantics
because APFloat does not have first-class support for bf16 types.

PiperOrigin-RevId: 225981289