Fix Clang private type alias parsing

[jeaye]

The LLVM issue for this is here: llvm/llvm-project#164885

Based on discussion with Vassil, the relevant code is here: https://github.com/llvm/llvm-project/blob/e04c01bcc91226d632f81bd3290c180deb0b6db8/clang/lib/Parse/ParseTentative.cpp#L18

This is a stack trace diff between clang-repl (left) and Clang (right) for when they get to the same access check: https://www.diffchecker.com/tQJVlTmv/

From this, Vassil infers that clang-repl is considering this as a statement and not a declaration.