[clang] [clang][Modules] Add explicit PrivateModuleFragmentDecl AST node (PR #223044)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> I was working on this and i found that ActOnGlobalModuleFragmentDecl also returns nullptr in all branches. Should I make PrivateModuleFragmentDecl into ModuleFragmentDecl and share between both functions?
> 
> @ChuanqiXu9

I didn't work on this because I didn't have a clear answer to the question "should we make a module decl a context?". As it looks like a context but I can image it will be a problem if we makes it a context as the context is critical in clang. And we didn't meet actual problem before.

Considering that bug report was generated by a fuzzer and actually an assertion, I prefer to fix that assertion of that control flow at that point instead of inserting the new AST node...

https://github.com/llvm/llvm-project/pull/223044