Allow non-literal static range pattern for match arms

Fix unintended error problem of:

static s: int = 1;
static e: int = 42;

fn main() {
    match 7 {
        s..e => (),
         ^~                 error: expected `=>` but found `..`
        _ => (),
    }
}
auto merge of #6245 : youknowone/rust/match-range-static, r=graydon

Fix unintended error problem of:
````
static s: int = 1;
static e: int = 42;

fn main() {
    match 7 {
        s..e => (),
         ^~                 error: expected `=>` but found `..`
        _ => (),
    }
}
````