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

[Mahmoud Ahmed via cfe-commits]

anondeveg 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 

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