Fixes for various endianness issues

[Hugobros3]

The SPIR-V specifications is agnostic on the issue of endianness (at least at the 32-bit word level), and tooling has some supporting code for it. It sadly appears to suffer from quite a bit of rot, this PR tackles the following:

• spirv-dis fails to disassemble a SPIR-V file in the non-native endianness
  • This is caused by trying to parse a string literal before the words are converted into native order, as assumed by MakeString
  • I changed it so that the whole file is byte-swapped when the parser is initialized, which opens the path for further simplifications
• source extraction code duplicates the logic added in mhillenbrand@a95b262 and fails to account for mismatched endianness
• Some amount of dead code resulting from the prior commit

I ran and passed all tests on a real PowerPC G5, which is as fun a way to spend a day as any other. There's probably more I can do, but I'd like this to be reviewed before just so I know I'm on the right track.

Additionally, I'm not sure how relevant big-endian SPIR-V support really is considering the state of the tooling. Rather it would probably be beneficial to have big-endian architectures default to emitting little-endian files, as I doubt there are many, if any implementations that correctly implement endianness agnosticism. I think it needs to at least be an option!

[CLAassistant]

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