[DirectX] Version in ProgramHeader is wrong on big endian host

[python3kgae]

MinorVersion and MajorVersion in llvm::dxbc::ProgramHeader used bitfield.
But the order of allocation of bit-field within a unit is implementation-defined.

Wrong version will be read on big endian host right now.

[bogner]

The SignatureElement struct in DXContainer.h also uses bitfields and should be looked at as well.

[bogner]

Note that this isn't necessarily that the wrong version will be read on a big endian host - it could potentially be read incorrectly on any host depending on the compiler.

From C++17's [class.bit]

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 ]