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.
It is meaningless to emit macro definitions for named modules. With some
small experiments, the size of the module for the named modules reduced
2%~4% after this patch.
It is helpful to know whih macro definition is emitted in the module
file without openning it directly. And this is not easy to be tested
with the lit test. So this patch add the facility to emit macro
definitions in `-module-file-info` action. And this should be innnocent
for every other cases.
