librustc: Resolve regions and report errors in trait/impl method

[pcwalton]

matching.

This breaks code like:

struct Foo<'a,'b> {
    x: &'a int,
    y: &'b int,
}

trait Tr {
    fn foo(x: Self) {}
}

impl<'a,'b> Tr for Foo<'a,'b> {
    fn foo(x: Foo<'b,'a>) {} // <-- bad
}

Change this code to not contain a lifetime mismatch error. For example:

struct Foo<'a,'b> {
    x: &'a int,
    y: &'b int,
}

trait Tr {
    fn foo(x: Self) {}
}

impl<'a,'b> Tr for Foo<'a,'b> {
    fn foo(x: Foo<'a,'b>) {} // OK
}

Closes #15517.

[breaking-change]

r? @alexcrichton