[Clang] Fix parsing of pack-index-specifier with template-id pattern

[sdkrystian]

Previously, ParseOptionalCXXScopeSpecifier only handled pack-index-specifiers where the pattern was a simple identifier (e.g., T...[0]::). This patch adds support for template-id patterns (e.g., Foo<T>...[0]::).

The fix involves two parts:

1. Extend ParsePackIndexingType to handle tok::annot_template_id in addition to tok::identifier.
2. Add handling inside the loop in ParseOptionalCXXScopeSpecifier for annot_template_id followed by ... [ constant-expression ].

This fixes code like:

  template<template<class...> class... L> void f() {
    using T = mp_quote<L>...[0]::template fn<void>;
  }

which previously failed to parse.

[mizvekov]

This needs rewiring for nested name and elaborated keyword support, with additional tests covering those cases.

AST dump tests and ast-print roundtrip tests would make good confirmation this works.

[Endilll]

1. I see that pack indexing is handled before the loop in ParseOptionalCXXScopeSpecifier, too:

   clang/clang/lib/Parse/ParseExprCXX.cpp

   Line 194 in b94c5e0

   SourceLocation EndLoc = ParsePackIndexingType(DS);

   
   I'm not sure we need to touch it, but would be nice if description touched on that

2. Would be nice to mention somewhere that previously only the identifier form of typedef-name within pack-index-specifier was handled, but this PR handles simple-template-id form of typedef-name.

3. "template-id" in the title mislead me to believe that this is covering not just simple-template-id, but also operator-function-id < template-argument-list (opt)> and literal-operator-id < template-argument-list (opt) >. This is not the case, and it would be nice to have couple more tests that make sure those forms of template-id are not accepted within pack-index-specifier.