asm: When pretty-printing, don't escape characters twice

pprust uses `print_string` to write out the template string, and
`print_string` already calls `escape_debug`, so `impl fmt::Display for
InlineAsmTemplatePiece` shouldn't do an additional `escape_debug`.

This fixes a pretty-printing bug that translated
`asm!("...\n...")`
to
`asm!("...\\n...")`

src/librustc_ast/ast.rs

@@ -1914,7 +1914,7 @@ impl fmt::Display for InlineAsmTemplatePiece {
                     match c {
                         '{' => f.write_str("{{")?,
                         '}' => f.write_str("}}")?,
-                        _ => write!(f, "{}", c.escape_debug())?,
+                        _ => c.fmt(f)?,
                     }
                 }
                 Ok(())