Close https://github.com/llvm/llvm-project/issues/72383
The implementation rationale is, I don't want to pass
`-fmodules-embed-all-files` all the time since we can't test it in lit
tests (we're using `clang_cc1`). So I tried to set it in FrontendActions
for modules.
Reduced BMI
Mitigate https://github.com/llvm/llvm-project/issues/61447
The root cause of the above problem is that when we write a declaration,
we need to lookup all the redeclarations in the imported modules. Then
it will be pretty slow if there are too many redeclarations in different
modules. This patch doesn't solve the porblem.
What the patchs mitigated is, when we writing a named module, we shouldn't
write the declarations from GMF if it is unreferenced **in current
module unit**. The difference here is that, if the declaration is used
in the imported modules, we used to emit it as an update. But we
definitely want to avoid that after this patch.
For that reproducer in
https://github.com/llvm/llvm-project/issues/61447, it used to take 2.5s
to compile and now it only takes 0.49s to compile, which is a big win.
