Add Clang attribute to ensure that fields are initialized explicitly (#102040)

This is a new Clang-specific attribute to ensure that field
initializations are performed explicitly.

For example, if we have
```
struct B {
  [[clang::explicit]] int f1;
};
```
then the diagnostic would trigger if we do `B b{};`:
```
field 'f1' is left uninitialized, but was marked as requiring initialization
```

This prevents callers from accidentally forgetting to initialize fields,
particularly when new fields are added to the class.

1 files changed, 2 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index fd3a5ec..704cb82 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -19232,6 +19232,8 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
         if (FT.hasNonTrivialToPrimitiveCopyCUnion() || Record->isUnion())
           Record->setHasNonTrivialToPrimitiveCopyCUnion(true);
       }
+      if (FD->hasAttr<ExplicitInitAttr>())
+        Record->setHasUninitializedExplicitInitFields(true);
       if (FT.isDestructedType()) {
         Record->setNonTrivialToPrimitiveDestroy(true);
         Record->setParamDestroyedInCallee(true);