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.
A step to address https://github.com/llvm/llvm-project/issues/62707.
It is not user friendly enough to drop the implicitly generated path
directly. Let's emit the warning first and drop it in the next version.
Close https://github.com/llvm/llvm-project/issues/56916
Within C++20 modules, we may have multiple same constructors in
multiple same RecordDecls. And it doesn't make sense naturally to create
duplicated deduction guides for the duplicated constructors.
