[clang] [Clang] prevent setting default lexical access specifier for missing primary declarations (PR #112424)

[Shafik Yaghmour via cfe-commits]

================
@@ -39,7 +39,8 @@ bool Sema::SetMemberAccessSpecifier(NamedDecl *MemberDecl,
                                     AccessSpecifier LexicalAS) {
   if (!PrevMemberDecl) {
     // Use the lexical access specifier.
-    MemberDecl->setAccess(LexicalAS);
----------------
shafik wrote:

I think these changes make sense, the diagnostic on forward reference is not great but that feels like a seperate already existing problem and not directly related to the crash.

Please update the summary. I also had one question on why we are checking `Name` as well.

https://github.com/llvm/llvm-project/pull/112424