[clang] [NFC] [Serializer] Pack information in serializer (PR #69287)

[Chuanqi Xu via cfe-commits]

================
@@ -617,24 +617,27 @@ void ASTDeclReader::VisitDecl(Decl *D) {
                            Reader.getContext());
   }
   D->setLocation(ThisDeclLoc);
-  D->InvalidDecl = Record.readInt();
-  if (Record.readInt()) { // hasAttrs
+
+  uint64_t DeclBits = Record.readInt();
+  D->InvalidDecl = DeclBits & 0x1;
+  D->setImplicit(DeclBits & (1 << 2));
+  D->Used = (DeclBits >> 3) & 0x1;
+  IsDeclMarkedUsed |= D->Used;
+  D->setReferenced(DeclBits & (1 << 4));
+  D->setTopLevelDeclInObjCContainer(DeclBits & (1 << 5));
+  D->setAccess((AccessSpecifier)((DeclBits >> 6) & 0x3));
+  D->FromASTFile = true;
+  auto ModuleOwnership = (Decl::ModuleOwnershipKind)((DeclBits >> 8) & 0x7);
----------------
ChuanqiXu9 wrote:

What I had in mind is to introduce a BitPacker, but it can't avoid magic constants completely. e.g, we have to know that the width of `AccessSpecifier ` should be 2 and the width of `ModuleOwnershipKind` should be 3.

Then we can refactor the above section to:

```
BitPacker Bits = Record.readInt();
D->InvalidDecl = Bits.getNextBit();
D->setImplicit(Bits.getNextBit());
D->Used = Bits.getNextBit();
IsDeclMarkedUsed |= D->Used;
D->setReferenced(Bits.getNextBit());
D->setTopLevelDeclInObjCContainer(Bits.getNextBit());
D->setAccess((AccessSpecifier)Bits.getNextBit</*width = */2>()); // Still we can't avoid the magic constants completely.
D->FromASTFile = true;
auto ModuleOwnership = (Decl::ModuleOwnershipKind)Bits.getNextBit</*width = */3>());
```

Would you be happy about the above form?

https://github.com/llvm/llvm-project/pull/69287