librustc: Forbid partial reinitialization of uninitialized structures or

librustc: Forbid partial reinitialization of uninitialized structures or
enumerations that implement the `Drop` trait.

This breaks code like:

    struct Struct {
        f: String,
        g: String,
    }

    impl Drop for Struct { ... }

    fn main() {
        let x = Struct { ... };
        drop(x);
        x.f = ...;
    }

Change this code to not create partially-initialized structures. For
example:

    struct Struct {
        f: String,
        g: String,
    }

    impl Drop for Struct { ... }

    fn main() {
        let x = Struct { ... };
        drop(x);
        x = Struct {
            f: ...,
            g: ...,
        }
    }

Closes #18571.

[breaking-change]

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(Joint authorship by pcwalton and Ryman; thanks all!)