[clang] [Clang][Parser] Fix crash of clang when using C++ constructs like :: in C code (PR #74926)

[via cfe-commits]

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@@ -2679,6 +2679,8 @@ Parser::ParseCXXClassMemberDeclaration(AccessSpecifier AS,
                                        ParsedAttributes &AccessAttrs,
                                        const ParsedTemplateInfo &TemplateInfo,
                                        ParsingDeclRAIIObject *TemplateDiags) {
+  assert(getLangOpts().CPlusPlus &&
+         "Call sites of this function should be guarded by checking for C++");
----------------
cor3ntin wrote:

```suggestion
         "ParseCXXClassMemberDeclaration should only be called in C++ mode");
```

https://github.com/llvm/llvm-project/pull/74926