clif: Fix parsing the `cold` calling convention (#8854)

The identifier for the `cold` calling convention overlaps with the
`cold` keyword for basic blocks so handle another kind of token when
parsing signatures.

cranelift/filetests/filetests/parser/cold.clif

@@ -0,0 +1,12 @@
+test cat
+
+function %cold() cold {
+    sig0 = () cold
+    sig1 = () -> i8 cold
+block1:
+    return
+}
+
+; sameln: function %cold() cold {
+; nextln:     sig0 = () cold
+; nextln:     sig1 = () -> i8 cold

cranelift/reader/src/parser.rs

@@ -1347,14 +1347,20 @@ impl<'a> Parser<'a> {
         }
 
         // The calling convention is optional.
-        if let Some(Token::Identifier(text)) = self.token() {
-            match text.parse() {
+        match self.token() {
+            Some(Token::Identifier(text)) => match text.parse() {
                 Ok(cc) => {
                     self.consume();
                     sig.call_conv = cc;
                 }
                 _ => return err!(self.loc, "unknown calling convention: {}", text),
+            },
+
+            Some(Token::Cold) => {
+                self.consume();
+                sig.call_conv = CallConv::Cold;
             }
+            _ => {}
         }
 
         Ok(sig)