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, 1 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index ee72dd8..95abd75 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -832,6 +832,7 @@ RedeclarableResult ASTDeclReader::VisitRecordDeclImpl(RecordDecl *RD) {
       RecordDeclBits.getNextBit());
   RD->setHasNonTrivialToPrimitiveDestructCUnion(RecordDeclBits.getNextBit());
   RD->setHasNonTrivialToPrimitiveCopyCUnion(RecordDeclBits.getNextBit());
+  RD->setHasUninitializedExplicitInitFields(RecordDeclBits.getNextBit());
   RD->setParamDestroyedInCallee(RecordDeclBits.getNextBit());
   RD->setArgPassingRestrictions(
       (RecordArgPassingKind)RecordDeclBits.getNextBits(/*Width=*/2));