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, 3 insertions, 1 deletions

diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index f8ed155..3b357f3 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -611,6 +611,7 @@ void ASTDeclWriter::VisitRecordDecl(RecordDecl *D) {
   RecordDeclBits.addBit(D->hasNonTrivialToPrimitiveDefaultInitializeCUnion());
   RecordDeclBits.addBit(D->hasNonTrivialToPrimitiveDestructCUnion());
   RecordDeclBits.addBit(D->hasNonTrivialToPrimitiveCopyCUnion());
+  RecordDeclBits.addBit(D->hasUninitializedExplicitInitFields());
   RecordDeclBits.addBit(D->isParamDestroyedInCallee());
   RecordDeclBits.addBits(llvm::to_underlying(D->getArgPassingRestrictions()), 2);
   Record.push_back(RecordDeclBits);
@@ -2480,7 +2481,8 @@ void ASTWriter::WriteDeclAbbrevs() {
             // isNonTrivialToPrimitiveCopy, isNonTrivialToPrimitiveDestroy,
             // hasNonTrivialToPrimitiveDefaultInitializeCUnion,
             // hasNonTrivialToPrimitiveDestructCUnion,
-            // hasNonTrivialToPrimitiveCopyCUnion, isParamDestroyedInCallee,
+            // hasNonTrivialToPrimitiveCopyCUnion,
+            // hasUninitializedExplicitInitFields, isParamDestroyedInCallee,
             // getArgPassingRestrictions
   // ODRHash
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 26));