[Parse] Disallow space between attribute name and '(' in Swift 6 mode

This allows us to resolve disambiguities of whether a parenthesis belong to an argument to the attribute or if it is eg. the start of a tuple.

include/swift/AST/DiagnosticsParse.def

@@ -1514,6 +1514,9 @@ ERROR(decl_attribute_applied_to_type,none,
 ERROR(attr_extra_whitespace_after_at,PointsToFirstBadToken,
       "extraneous whitespace between '@' and attribute name", ())
 
+ERROR(attr_extra_whitespace_before_lparen,PointsToFirstBadToken,
+      "extraneous whitespace between attribute name and '('", ())
+
 ERROR(attr_expected_lparen,none,
 "expected '(' in '%0' %select{attribute|modifier}1", (StringRef, bool))
 

include/swift/Parse/Parser.h

@@ -602,6 +602,18 @@ public:
     return true;
   }
 
+  /// Consume a '('. If it is not directly following the previous token, emit an
+  /// error (Swift 6) or warning (Swift <6) that attribute name and parentheses
+  /// must not be separated by a space.
+  SourceLoc consumeAttributeLParen();
+
+  /// If the next token is a '(' that's not on a new line consume it, and error
+  /// (Swift 6) or warn (Swift <6) that the attribute must not be separted from
+  /// the '(' by a space.
+  ///
+  /// If the next token is not '(' or it's on a new line, return false.
+  bool consumeIfAttributeLParen();
+
   bool consumeIfNotAtStartOfLine(tok K) {
     if (Tok.isAtStartOfLine()) return false;
     return consumeIf(K);