MCDC coverage: support nested decision coverage
#123409 provided the initial MCDC coverage implementation.
As referenced in #124144, it does not currently support "nested" decisions, like the following example :
```rust
fn nested_if_in_condition(a: bool, b: bool, c: bool) {
if a && if b || c { true } else { false } {
say("yes");
} else {
say("no");
}
}
```
Note that there is an if-expression (`if b || c ...`) embedded inside a boolean expression in the decision of an outer if-expression.
This PR proposes a workaround for this cases, by introducing a Decision context stack, and by handing several `temporary condition bitmaps` instead of just one.
When instrumenting boolean expressions, if the current node is a leaf condition (i.e. not a `||`/`&&` logical operator nor a `!` not operator), we insert a new decision context, such that if there are more boolean expressions inside the condition, they are handled as separate expressions.
On the codegen LLVM side, we allocate as many `temp_cond_bitmap`s as necessary to handle the maximum encountered decision depth.
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