librustc: Implement lifetime elision.

This implements RFC 39. Omitted lifetimes in return values will now be
inferred to more useful defaults, and an error is reported if a lifetime
in a return type is omitted and one of the two lifetime elision rules
does not specify what it should be.

This primarily breaks two uncommon code patterns. The first is this:

    unsafe fn get_foo_out_of_thin_air() -> &Foo {
        ...
    }

This should be changed to:

    unsafe fn get_foo_out_of_thin_air() -> &'static Foo {
        ...
    }

The second pattern that needs to be changed is this:

    enum MaybeBorrowed<'a> {
        Borrowed(&'a str),
        Owned(String),
    }

    fn foo() -> MaybeBorrowed {
        Owned(format!("hello world"))
    }

Change code like this to:

    enum MaybeBorrowed<'a> {
        Borrowed(&'a str),
        Owned(String),
    }

    fn foo() -> MaybeBorrowed<'static> {
        Owned(format!("hello world"))
    }

Closes #15552.

[breaking-change]

r? @nick29581
nit: this comment should have a "Second, ..." clause which states where the other rule is applied.