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.
Closes https://github.com/llvm/llvm-project/issues/54457.
This removes a FIXME we found previously. But we didn't remove the FIXME
that time due to the lack of the corresponding test. And now we found
the corresponding test so we can remove it.
