Fix "Quasi-quoting is inefficient" warning in incremental rustbuild.

After #43252 is merged, building stage0 libcore with -i (--incremental)
flag will cause 17 "Quasi-quoting might make incremental compilation very
inefficient: NtExpr(..)" warnings, as in #40946.

Fixing the warning in #40946 will take 12 weeks from now to make into the
next stage0, so it is quicker to workaround it in libcore instead.

src/libcore/default.rs

@@ -126,7 +126,7 @@ pub trait Default: Sized {
 }
 
 macro_rules! default_impl {
-    ($t:ty, $v:expr, $doc:expr) => {
+    ($t:ty, $v:expr, $doc:tt) => {
         #[stable(feature = "rust1", since = "1.0.0")]
         impl Default for $t {
             #[inline]