Add noalias to safe shared reference parameters

This add the LLVM noalias attribute to parameters of a
shared reference type (&) which have a safe interior.
rustc: Compare paths with --extern, not bytes

The right hand side of the comparison in these checks are values of type
Option<&Path> which are normalized versions of the left-hand side, so they're
not guaranteed to be byte-for-byte equivalent even though they're the same path.

For this reasons, the command line arguments are promoted to paths for
comparison of equality.

This fixes a bug on windows where if a library was specified with --extern it
would then be picked up twice because it was not considered to have been
previously registered.
librustc: Check built-in trait bounds on implementations when direct
method calls are involved.

This breaks code like:

    impl<T:Copy> Foo for T { ... }

    fn take_param<T:Foo>(foo: &T) { ... }

    fn main() {
        let x = box 3i; // note no `Copy` bound
        take_param(&x);
    }

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

    impl<T:Copy> Foo for T { ... }

    fn take_param<T:Foo>(foo: &T) { ... }

    fn main() {
        let x = 3i; // satisfies `Copy` bound
        take_param(&x);
    }

Closes #15860.

[breaking-change]
auto merge of #15957 : pcwalton/rust/builtin-bound-impl-checking, r=huonw,pnkfelix

method calls are involved.

This breaks code like:

    impl<T:Copy> Foo for T { ... }

    fn take_param<T:Foo>(foo: &T) { ... }

    fn main() {
        let x = box 3i; // note no `Copy` bound
        take_param(&x);
    }

Change this code to not contain a type error. For example:

    impl<T:Copy> Foo for T { ... }

    fn take_param<T:Foo>(foo: &T) { ... }

    fn main() {
        let x = 3i; // satisfies `Copy` bound
        take_param(&x);
    }

Closes #15860.

[breaking-change]

r? @alexcrichton
auto merge of #15970 : Zoxc/rust/noalias-ref, r=cmr

This add the LLVM noalias attribute to parameters of a
shared reference type (&) which have a safe interior.
auto merge of #15971 : alexcrichton/rust/hurray-for-windows, r=pcwalton

The right hand side of the comparison in these checks are values of type
Option<&Path> which are normalized versions of the left-hand side, so they're
not guaranteed to be byte-for-byte equivalent even though they're the same path.

For this reasons, the command line arguments are promoted to paths for
comparison of equality.

This fixes a bug on windows where if a library was specified with --extern it
would then be picked up twice because it was not considered to have been
previously registered.