fix overflow during type inference for tuple struct patterns

[lowr]

The following code causes integer overflow during type inference for (malformed) tuple struct patterns.

struct S(usize);
let S(.., a, b) = S(1);

It has been panicking only in debug builds, and working in a way in release builds but it was inconsistent with type inference for tuple patterns:

struct S(usize);
let S(.., a, b) = S(1); // a -> unknown, b -> usize
let (.., a, b) = (1,);  // a -> usize, b -> unknown

With this PR, the overflow no longer happens by utilizing saturating_sub() like in other places and type inference for tuple struct patterns is in line with that for tuple patterns.

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit be2fa2b has been approved by Veykril

[bors]

⌛ Testing commit be2fa2b with merge f94fa62...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing f94fa62 to master...