[ABI] Leading :: changes mangling

```C++
template ,bool> = true>
void foo() {}

int a = foo();
```

and

```C++
template ,bool> = true>
void foo() {}

int a = foo();
```
https://godbolt.org/z/9cY5TdesM

will produce different mangling.

They first will produce _Z3fooIKiTnNSt9enable_ifIXsr3stdE10is_const_vIT_EEbE4typeELb1EEvv and the second will produce _Z3fooIKiTnNSt9enable_ifIXgssr3stdE10is_const_vIT_EEbE4typeELb1EEvv

This also introduces an ABI breaking with GCC as GCC produces _Z3fooIKiLb1EEvv

Reproduce both examples using the linked Compiler Explorer case and compare the emitted manglings with GCC's output. Investigate the ABI mangling rules for a leading `::` in the constraint expression; done means the compiler's behavior is consistently defined and no longer breaks ABI compatibility with GCC.

Written by the indexing model from the issue text.