Wrap contract clauses in brances instead of parenthesis

The compiler complained about uncecessary parenthesis on contract clauses,
which were insterted by the contract macros. This commit changes the
macro to use braces as the delimiter instead, fixing the issue.

Author: dawidl022

compiler/rustc_builtin_macros/src/contracts.rs

@@ -141,7 +141,7 @@ fn expand_requires_tts(
         new_tts.push_tree(TokenTree::Delimited(
             DelimSpan::from_single(attr_span),
             DelimSpacing::new(Spacing::JointHidden, Spacing::JointHidden),
-            token::Delimiter::Parenthesis,
+            token::Delimiter::Brace,
             annotation,
         ));
         Ok(())
@@ -163,7 +163,7 @@ fn expand_ensures_tts(
         new_tts.push_tree(TokenTree::Delimited(
             DelimSpan::from_single(attr_span),
             DelimSpacing::new(Spacing::JointHidden, Spacing::JointHidden),
-            token::Delimiter::Parenthesis,
+            token::Delimiter::Brace,
             annotation,
         ));
         Ok(())