comp: Fix alignment of bitfields in some edge cases.

In order to properly compute the size of the bitfield for overaligned
fields, we need to know the offset in the struct, not just in the
allocation unit.

E.g., something like:

  char; // Byte 1
  char: 8; // Byte 2
  short: 16; // Bytes 3 and 4

Should align differently than:

  char: 8; // Byte 1
  short: 16; // Bytes 3 and 4

If we can't find the start offset in the struct, we fall back to our
previous behavior (this can happen with C++ templates for example).

We remove some code related to `is_ms_struct`, which was effectively
dead. If someone cares about it they can resurrect it.

We also don't align per-bitfield unit, but in order to mostly preserve
behavior, we keep inserting a field at the beginning of the struct
rather than using repr(align). Otherwise we hit #2179 in cases where we
did not use to.

We might want to extend that approach to repr(align) stuff, in order to
try to fix #2179 more often, but for now do the minimal change.

Fixes #1377
Fixes #3105
Fixes #743
Fixes #981
Fixes #1314

Co-authored-by: qinghon <[email protected]>

Author: emilio