coverage: Discard spans that fill the entire function body

When we try to extract coverage-relevant spans from MIR, sometimes we see MIR
statements/terminators whose spans cover the entire function body. Those spans
tend to be unhelpful for coverage purposes, because they often represent
compiler-inserted code, e.g. the implicit return value of `()`.

tests/coverage/unicode.coverage

@@ -29,8 +29,7 @@
    LL|       |
    LL|       |macro_rules! macro_that_defines_a_function {
    LL|       |    (fn $名:ident () $体:tt) => {
-   LL|      1|        fn $名 () $体 fn 他 () {}
-                                      ^0
+   LL|      0|        fn $名 () $体 fn 他 () {}
    LL|       |    }
    LL|       |}
    LL|       |