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/62589.
Previously, for global module fragments, we will justify if it is
visible by the visibility of their top level parents. But this is an
overkill, it is problematic if we have a deduction guide in the global
module fragments. See the attached test for example. In this example, we
will mark the global module fragments as visible, but our old strategy
will miss the case surprisingly due to we will only search for the top
level modules.
