Constraint solver: use canBeArgumentLabel to determine when to quote a label.

Thanks to Chris for noticing this a few months back.

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -21,6 +21,7 @@
 #include "swift/AST/ASTVisitor.h"
 #include "swift/AST/ASTWalker.h"
 #include "swift/AST/Attr.h"
+#include "swift/Basic/StringExtras.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/SmallString.h"
@@ -5460,9 +5461,7 @@ diagnoseArgumentLabelError(Expr *expr, ArrayRef<Identifier> newNames,
       continue;
     }
 
-    tok newNameKind =
-      Lexer::kindOfIdentifier(newName.str(), /*inSILMode=*/false);
-    bool newNameIsReserved = newNameKind != tok::identifier;
+    bool newNameIsReserved = !canBeArgumentLabel(newName.str());
     llvm::SmallString<16> newStr;
     if (newNameIsReserved)
       newStr += "`";

test/Constraints/keyword_arguments.swift

@@ -51,7 +51,7 @@ allkeywords1(1, y: 2) // expected-error{{missing argument label 'x:' in call}} {
 
 // If keyword is reserved, make sure to quote it. rdar://problem/21392294
 func reservedLabel(x: Int, `repeat`: Bool) {}
-reservedLabel(1, true) // expected-error{{missing argument label 'repeat:' in call}}{{18-18=`repeat`: }}
+reservedLabel(1, true) // expected-error{{missing argument label 'repeat:' in call}}{{18-18=repeat: }}
 
 // Insert missing keyword before initial backtick. rdar://problem/21392294 part 2
 func reservedExpr(x: Int, y: Int) {}