[clang] [CIR] Add bitfield offset calculation for big-endian targets (PR #145067)

[Razvan Lupusoru via cfe-commits]

razvanlupusoru wrote:

@Andres-Salamanca Are you expecting the test to be failing?

Here is what I am seeing:
```
error: ClangIR code gen Not Yet Implemented: NYI AAPCS bit-fields
error: ClangIR code gen Not Yet Implemented: NYI AAPCS bit-fields
error: ClangIR code gen Not Yet Implemented: NYI AAPCS bit-fields
3 errors generated.

--

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Failed Tests (1):
  Clang :: CIR/CodeGen/bitfields_be.c
```

https://github.com/llvm/llvm-project/pull/145067