[Clang] Clang accepts NonIdentifier string in the `gnu::abi_tag` attribute

[da-viper]

// main.cpp 

[[gnu::abi_tag("[[something]]")]]
int get_int() { 
	return 0;
}

int main() { 
	int ret = get_int();
	return 0;
}

with g++

$>  g++ test.cpp
test.cpp:3:13: error: arguments to the ‘abi_tag’ attribute must contain valid identifiers
    3 | int get_int() {
      |             ^
test.cpp:3:13: note: ‘[’ is not a valid first character for an identifier

with clang (this should be an error). As I assume we need to match gcc as it is a gnu extension

$>  clang++ test.cpp
$>

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Ebuka Ezike (da-viper)

```cpp // main.cpp

[[gnu::abi_tag("[[something]]")]]
int get_int() {
return 0;
}

int main() {
int ret = get_int();
return 0;
}

with clang 

$> clang++ test.cpp
$>

with g++ 

$> g++ test.cpp
test.cpp:3:13: error: arguments to the ‘abi_tag’ attribute must contain valid identifiers
3 | int get_int() {
| ^
test.cpp:3:13: note: ‘[’ is not a valid first character for an identifier

</details>

[shafik]

I am guessing this was not intentional: https://godbolt.org/z/1rboeYsrv