Reimplement bitfield access unit computation for SysV ABIs. Considers
expense of unaligned and non-power-of-2 accesses.

At the moment,distinct fields for each union member are generated. When
copying a union, we don't know which union member is active, so there's
no benefit from recording the different fields. It can result in
converting tbaa.struct fields to incorrect tbaa nodes when extracting
fields.

PR: https://github.com/llvm/llvm-project/pull/84370

Adds extra test coverage for TBAA generation for copies of structs with
union members.

At the moment, clang generates what I believe are incorrect !tbaa.struct
fields for named bitfields. At the moment, the base type size is used
for named bifields (e.g. sizeof(int)) instead of the bifield width per
field. This results in overalpping fields in !tbaa.struct metadata.

This causes incorrect results when extracting individual copied fields
from !tbaa.struct as in added in dc85719d5.

This patch fixes that by skipping by combining adjacent bitfields
in fields with correct sizes.

Fixes https://github.com/llvm/llvm-project/issues/82586

Extra tests with unnamed bitfields for
https://github.com/llvm/llvm-project/pull/82922.

Additional test for https://github.com/llvm/llvm-project/pull/82922/.

Add test for tbaa.struct metadata creation for copies of a struct with
named bitfields.

Test for https://github.com/llvm/llvm-project/issues/82586.

Conversion performed using the script at:
https://gist.github.com/nikic/98357b71fd67756b0f064c9517b62a34

These are only tests where no manual fixup was required.

This adds -no-opaque-pointers to clang tests whose output will
change when opaque pointers are enabled by default. This is
intended to be part of the migration approach described in
https://discourse.llvm.org/t/enabling-opaque-pointers-by-default/61322/9.

The patch has been produced by replacing %clang_cc1 with
%clang_cc1 -no-opaque-pointers for tests that fail with opaque
pointers enabled. Worth noting that this doesn't cover all tests,
there's a remaining ~40 tests not using %clang_cc1 that will need
a followup change.

Differential Revision: https://reviews.llvm.org/D123115

Fixed Clang tests.