Implement DR277 to reject non-object declarations in for loops pre-C23

[Halalaluyafail3]

Clang incorrectly allows the following for loop:

for (enum fred { jim, sheila = 10 } i = jim; i < sheila; i++);

This is invalid and is mentioned specifically in DR277.

Also, I think it is worth specifically mentioning object declarations in structures with no tag here e.g.:

for(struct{int x;}x;0;);

GCC accepts this and I think it is fine, since members are objects.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Halalaluyafail3)

Clang incorrectly allows the following for loop: ```c for (enum fred { jim, sheila = 10 } i = jim; i < sheila; i++); ``` This is invalid and is mentioned specifically in [DR277](https://www.open-std.org/jtc1/sc22/wg14/issues/c99/issue0277.html).

Also, I think it is worth specifically mentioning object declarations in structures with no tag here e.g.:

for(struct{int x;}x;0;);

GCC accepts this and I think it is fine, since members are objects.

[llvmbot]

@llvm/issue-subscribers-c

Author: None (Halalaluyafail3)

Clang incorrectly allows the following for loop: ```c for (enum fred { jim, sheila = 10 } i = jim; i < sheila; i++); ``` This is invalid and is mentioned specifically in [DR277](https://www.open-std.org/jtc1/sc22/wg14/issues/c99/issue0277.html).

Also, I think it is worth specifically mentioning object declarations in structures with no tag here e.g.:

for(struct{int x;}x;0;);

GCC accepts this and I think it is fine, since members are objects.

[shafik]

CC @AaronBallman