[Clang] Recurse into parsing when using pack-indexing as a specifier (#119123)

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

clang/docs/ReleaseNotes.rst

@@ -795,6 +795,7 @@ Bug Fixes to C++ Support
 - Fixed a bug where bounds of partially expanded pack indexing expressions were checked too early. (#GH116105)
 - Fixed an assertion failure caused by using ``consteval`` in condition in consumed analyses. (#GH117385)
 - Fix a crash caused by incorrect argument position in merging deduced template arguments. (#GH113659)
+- Fixed a parser crash when using pack indexing as a nested name specifier. (#GH119072) 
 - Fixed an assertion failure caused by mangled names with invalid identifiers. (#GH112205)
 - Fixed an incorrect lambda scope of generic lambdas that caused Clang to crash when computing potential lambda
   captures at the end of a full expression. (#GH115931)

clang/lib/Parse/ParseExpr.cpp

@@ -1199,7 +1199,7 @@ ExprResult Parser::ParseCastExpression(CastParseKind ParseKind,
         // If the token is not annotated, then it might be an expression pack
         // indexing
         if (!TryAnnotateTypeOrScopeToken() &&
-            Tok.is(tok::annot_pack_indexing_type))
+            Tok.isOneOf(tok::annot_pack_indexing_type, tok::annot_cxxscope))
           return ParseCastExpression(ParseKind, isAddressOfOperand, isTypeCast,
                                      isVectorLiteral, NotPrimaryExpression);
       }

clang/test/Parser/cxx2c-pack-indexing.cpp

@@ -74,3 +74,12 @@ struct SS {
     }
 };
 }
+
+namespace GH119072 {
+
+template<typename... Ts>
+void foo() {
+  decltype(Ts...[0]::t) value;
+}
+
+}