libsyntax: Forbid type parameters in field expressions.

This breaks code like:

    struct Foo {
        x: int,
    }

    let f: Foo = ...;
    ... f.x::<int> ...

Change this code to not contain an unused type parameter. For example:

    struct Foo {
        x: int,
    }

    let f: Foo = ...;
    ... f.x ...

Closes #18680.

[breaking-change]
auto merge of #18879 : pcwalton/rust/path-silliness, r=aturon

This breaks code like:

    struct Foo {
        x: int,
    }

    let f: Foo = ...;
    ... f.x::<int> ...

Change this code to not contain an unused type parameter. For example:

    struct Foo {
        x: int,
    }

    let f: Foo = ...;
    ... f.x ...

Closes #18680.

[breaking-change]

r? @aturon