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

[Shafik Yaghmour via cfe-commits]

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

Please also add more details to the summary, it is helpful to have some idea what you think the underlying problem is and why this fix is correct before reviewing. 

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