Handle GCC's write-only inline asm constraint in liveness, borrowck and trans.

[eddyb]

I've implemented analysis support for the GCC '=' write-only inline asm constraint modifier. I had more changes, for '+' (read+write) as well, but it turns out LLVM doesn't support '+' at all.

I've removed the need for wrapping each output in ExprAddrOf, as that would require unwrapping almost everywhere and it was harder to reason about in borrowck than ExprAssign's LHS.

With this change, rustc will treat (in respect to validity of accessing a local) code like this:

let x: int;
unsafe {
    asm!("mov $1, $0" : "=r"(x) : "r"(5u));
}

as if it were this:

let x : int;
x = 5;

Previously, the local was required to be both mutable and initialized, and the write effect wasn't recorded.

[alexcrichton]

It looks to be that we have a section of constraints which are all dedicated to "output constraints", but it's possible to have an output constraint which isn't validated to be writable? It seems to me that we should merge all the constraints into one list (having = denote whether it's input or output), or we should leave the lists separate and not require = at all (because it's implied by where it's located). If we need to add the constraint for LLVM, then we can to that at trans time.

Or is there a purpose for having an output constraint without an =?

[eddyb]

For the following C code:

int main() {
    short port = 0;
    int val = 0;
    asm("in %1, %0" : "a"(val) : "d"(port));
}

GCC gives me this error:

<stdin>: In function ‘main’:
<stdin>:4:5: error: output operand constraint lacks ‘=’
<stdin>:4:5: error: output operand constraint lacks ‘=’
<stdin>:4:5: error: invalid lvalue in asm output 0

while clang just fails validation, without a precise cause (but better location info than GCC):

<stdin>:4:23: error: invalid output constraint 'a' in asm
    asm("in %1, %0" : "a"(val) : "d"(port));}
                      ^
1 error generated.

We can follow the same rule, require '=' at the start of output constraints, and then remove all my .starts_with("=") conditions.

[luqmana]

This can just be one line, no need to manually wrap. The max line length is 100 characters.