Close https://github.com/llvm/llvm-project/issues/71034
See
https://discourse.llvm.org/t/rfc-c-20-modules-introduce-thin-bmi-and-decls-hash/74755
This patch introduces reduced BMI, which doesn't contain the definitions
of functions and variables if its definitions won't contribute to the
ABI.
Testing is a big part of the patch. We want to make sure the reduced BMI
contains the same behavior with the existing and relatively stable
fatBMI. This is pretty helpful for further reduction.
The user interfaces part it left to following patches to ease the
reviewing.
Classes can have implicit members that were added before fields were
deserialized. These members were previously silently removed from
`decls()` when fields were deserialized after them.
This was the root cause of a compilation error exposed in
bc95f27337c7ed77c28e713c855272848f01802a, added a test for it.
Reviewed By: ChuanqiXu
Differential Revision: https://reviews.llvm.org/D148515
