attributes: remove unnecessary quote_spanned!

## Motivation

Apparently, using `quote_spanned!` can trigger a Clippy bug where the
text `else`, even inside a comment, _may_ cause the
`suspicious_else_formatting` lint to be triggered incorrectly (see
rust-lang/rust-clippy#7760 and rust-lang/rust-clippy#6249). This causes
the lint to fire in some cases when the `#[instrument]` attribute is
used on `async fn`s. See issue #1613 for details.

## Solution

It turns out that some of the uses of `quote_spanned!` in the
`tracing-attributes` code generation are not needed. We really only need
`quote_spanned!` when actually interpolating the user provided code into
a block, not in the `tracing-attributes` code that inserts the generated
code for producing the span etc. Replacing some of these
`quote_spanned!` uses with the normal `quote!` macro still generates
correct location diagnostics for errors in the user code, but fixes the
incorrect clippy lint.

I've added a few test cases that should reproduce the bug.

Fixes #1613

tracing-attributes/src/lib.rs

@@ -532,7 +532,7 @@ fn gen_block(
             )
         };
 
-        return quote_spanned!(block.span()=>
+        return quote!(
             if tracing::level_enabled!(#level) {
                 let __tracing_attr_span = #span;
                 tracing::Instrument::instrument(
@@ -546,7 +546,7 @@ fn gen_block(
         );
     }
 
-    let span = quote_spanned!(block.span()=>
+    let span = quote!(
         // These variables are left uninitialized and initialized only
         // if the tracing level is statically enabled at this point.
         // While the tracing level is also checked at span creation
@@ -580,7 +580,7 @@ fn gen_block(
         );
     }
 
-    quote_spanned!(block.span()=>
+    quote_spanned!(block.span() =>
         // Because `quote` produces a stream of tokens _without_ whitespace, the
         // `if` and the block will appear directly next to each other. This
         // generates a clippy lint about suspicious `if/else` formatting.