[clang] [C++20] [Modules] Introduce reduced BMI (PR #75894)

[Chuanqi Xu via cfe-commits]

================
@@ -830,6 +843,19 @@ class PCHGenerator : public SemaConsumer {
   bool hasEmittedPCH() const { return Buffer->IsComplete; }
 };
 
+class ReducedBMIGenerator : public PCHGenerator {
+public:
+  ReducedBMIGenerator(const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
+                      StringRef OutputFile, std::shared_ptr<PCHBuffer> Buffer,
+                      bool IncludeTimestamps);
+
+  void HandleTranslationUnit(ASTContext &Ctx) override;
+};
+
+/// If the definition may impact the ABI. If yes, we're allowed to eliminate
+/// the definition of D in reduced BMI.
+bool MayDefAffectABI(const Decl *D);
----------------
ChuanqiXu9 wrote:

Nice catch up.

I changed the name to `CanElideDeclDef` and the comments to:

```
/// If we can elide the definition of \param D in reduced BMI.
///
/// Generally, we can elide the definition of a declaration if it won't affect the
/// ABI. e.g., the non-inline function bodies.
```

Hope this to be clear.

https://github.com/llvm/llvm-project/pull/75894