`void f(T... [N])` is no longer treated as a function with a parameter
of pack expansion type after the implementation of the pack indexing
feature. This patch introduces a warning to clarify such cases while
maintaining it as a pack indexing type in all language modes.
Closes https://github.com/llvm/llvm-project/issues/115222
Pack indexing type that introduces a scope, e.g. `T...[0]::value` would
be annotated as `annot_cxxscope`. This is something we didn't handle in
`ParseCastExpression`, causing it to mistakely fall through to the logic
for `raw_identifier`.
We should recurse into parsing the following specifiers for such cases.
Closes#119072
Implements https://isocpp.org/files/papers/P2662R3.pdf
The feature is exposed as an extension in older language modes.
Mangling is not yet supported and that is something we will have to do before release.
