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.
Close https://github.com/llvm/llvm-project/issues/60486.
When I look back at this problem again, it only appears if we specify it
with `-fmodule-file=<BMI-path>`. And it disappears after we specify it
as `-fmodule-file=<module-name>=<BMI-path>`. Since we want to depreacate
the form `-fmodule-file=<BMI-path>`, we can think the problem goes
away.