Continuing #5421

I believe this patch incorporates all expected syntax changes from extern
function reform (#3678). You can now write things like:

    extern "<abi>" fn foo(s: S) -> T { ... }
    extern "<abi>" mod { ... }
    extern "<abi>" fn(S) -> T

The ABI for foreign functions is taken from this syntax (rather than from an
annotation).  We support the full ABI specification I described on the mailing
list.  The correct ABI is chosen based on the target architecture.

Calls by pointer to C functions are not yet supported, and the Rust type of
crust fns is still *u8.

r? @graydon

Closes #5403

Removing it from the tree is an ordeal and there is no official way
to disable clang via LLVM's Makefiles so this edits the Makefile
in llvm/tools after running configure.

... by adding Kleene '+' in two places, and changing a "non-slash" into "non_slash_or_star".

Closes #1588

Before it wouldn't warn about unused imports in the list if something in the list was used. These commits fix that case, add a test, and remove all unused imports in lists of imports throughout the compiler.

libcore: language change minor fix for ARM & MIPS

fix context.rs 

r? @brson

r? @brson

re bug that @nikomatsakis was hitting: when you define a `static` (old: `const`) containing a `&` or `&[]` expression, it will create temporaries (the underlying pointee) by creating a throwaway symbol for each temporary, each with _global_ linkage, and each named `"const"`. LLVM will helpfully rename multiple copies of this throwaway symbol to `"const1"` and `"const2"` and so forth in the _same_ library. But if you have _2 libraries_ -- say, libcore and librustc -- that both do this, the dynamic linker (at least on linux) will happily do horrible things like make the slice in one library point to the bytes of the vector from the other library. This is obviously a recipe for much hilarity and head-scratching.

The solution is to change the linkage to something else, internal or (in the case of this patch) _private_.

It will require a snapshot to integrate this into stage0 and thereby fix the problem / unblock patches that were hitting this in stage1.

Before, if anything in a list was used, the entire list was considered to be
used. This corrects this and also warns on a span of the actual unused import
instead of the entire list.

@nikomatsakis pointed out that `fn read_option<T>(&self, f: &fn() -> T) -> Option<T>` should have this syntax so it can work with custom option types: `fn read_option<T>(&self, f: &fn(bool) -> T) -> T`.

Also, this also includes some `#[inline(always)]` on the memory functions in `src/libcore/unstable/lang.rs` to reduce one level of indirection when allocating memory.

Clean things up a bit. Also, allow selecting intel syntax in addition to the default AT&T dialect.