Merge #357

357: Create liveness analysis for dead code detection r=philberty a=thomasyonug

This is the very beginning work for dead code detection. #330 
1. create a new test for this feature.
2. handle an extremely simple case.

After compiling the rust1, and feed "testsuite/rust.test/xfail_compile/unused.rs" to it.
``` rust

fn bar() { // {dg-warning "function is never used: `bar`"}
    foo();
}

fn foo() { // {dg-warning "function is never used: `foo`"}
    bar();
}

fn f() {

}

fn main() {
    f();
}

```
we will get some warnings.
``` bash
../gccrs/gcc/testsuite/rust.test/xfail_compile/unused.rs:2:1: warning: function is never used: `[bar]`
    2 | fn bar() { // {dg-warning "function is never used: `bar`"}
      | ^
../gccrs/gcc/testsuite/rust.test/xfail_compile/unused.rs:6:1: warning: function is never used: `[foo]`
    6 | fn foo() { // {dg-warning "function is never used: `foo`"}
      | ^

```

Co-authored-by: Thomas Young <wenzhang5800@gmail.com>

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