[Clang] Accepts invalid auto on a declaration of class

[YuchangSu]

auto class X1 { } ;

See Compiler Explorer: https://godbolt.org/z/3WheK64E3
auto can only be specified for variables or function declarations.It is rejected by GCC, but accepted by Clang.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (YuchangSu)

``` auto class X1 { } ; ``` See Compiler Explorer: [https://godbolt.org/z/3WheK64E3](https://godbolt.org/z/3WheK64E3) auto can only be specified for variables or function declarations.It is rejected by GCC, but accepted by Clang.

[shafik]

This goes back a while: https://godbolt.org/z/P41x96f4x