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/macro_name_span.coverage

@@ -21,6 +21,6 @@
    LL|       |macro_name_span_helper::macro_that_defines_a_function! {
    LL|      1|    fn affected_function() {
    LL|      1|        macro_with_an_unreasonably_and_egregiously_long_name!();
-   LL|      1|    }
+   LL|       |    }
    LL|       |}