pprust: adjust mixed comment printing

This commit adjusts the pretty printing of mixed comments so that the
initial zero-break isn't emitted at the beginning of the line. Through
this, the `block-comment-wchar` test can have the `pp-exact` file
removed, as it no longer converges from pretty printing of the source.

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_ast_pretty/pprust.rs

@@ -450,7 +450,9 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
     fn print_comment(&mut self, cmnt: &comments::Comment) {
         match cmnt.style {
             comments::Mixed => {
-                self.zerobreak();
+                if !self.is_beginning_of_line() {
+                    self.zerobreak();
+                }
                 if let Some((last, lines)) = cmnt.lines.split_last() {
                     self.ibox(0);
 

src/test/pretty/block-comment-wchar.pp

@@ -73,15 +73,13 @@
     */
 
 
-
     /* */
 
     /*
       Hello from offset 6
       Space 6+2:                     compare A
       Ogham Space Mark 6+2: compare B
     */
-
     /* */
 
     /*