Add support for hexadecimal float literals

MLIR does not have support for parsing special floating point values such as
infinities and NaNs.  If programmatically constructed, these values are printed
as NaN and (+-)Inf and cannot be parsed back.  Add parser support for
hexadecimal literals in float attributes, following LLVM IR.  The literal
corresponds to the in-memory representation of the floating point value.
IEEE 754 defines a range of possible values for NaNs, storing the bitwise
representation allows MLIR to properly roundtrip NaNs with different bit values
of significands.

PiperOrigin-RevId: 260018802