Avoid using bitfield in dxbc::ProgramHeader.
It could potentially be read incorrectly on any host depending on the
compiler.
From [C++17's
[class.bit]](https://timsong-cpp.github.io/cppwp/n4659/class.bit#1)
> Bit-fields are packed into some addressable allocation unit. [ Note:
Bit-fields straddle allocation units on some machines and not on others.
Bit-fields are assigned right-to-left on some machines, left-to-right on
others. — end note ]
For #91793