[clang-repl] Fix disambiguation of out-of-line member with private types (#178842)

This patch fixes a bug in clang-repl where out-of-line member function
definitions were incorrectly identified as statements when they involved
private type aliases.

The issue occurred because `isCXXDeclarationStatement` would trigger
immediate access checks during tentative parsing. Since the context of
an out-of-line definition isn't fully established during this phase,
Sema would incorrectly flag private members as inaccessible, causing
the parser to fail the declaration check and fall back to statement
parsing.

Changes:
- In `isCXXDeclarationStatement`, use `TentativeParsingAction` to
  ensure the token stream is fully restored.
- Use `SuppressAccessChecks` during the tentative disambiguation phase
  to prevent premature access errors.
- Ensure that formal access verification still occurs during the
  actual parsing phase in `ParseDeclarationOrFunctionDefinition`.
- Add tests and resolve a FIXME in
  clang/test/Interpreter/disambiguate-decl-stmt.cpp.

Fixes llvm#164885.
See also jank-lang/jank#609.