Parser: Reject @c attributes using the string format

Reject `@c` attributes in the format of `@c("customName")` to accept
only `@c(customName)` and of course the bare `@c`.

Author: xymus

lib/Parse/ParseDecl.cpp

@@ -3027,14 +3027,7 @@ ParserStatus Parser::parseNewDeclAttribute(DeclAttributes &Attributes,
   }
 
   case DeclAttrKind::CDecl: {
-    if (!AttrName.starts_with("_") &&
-
-        // Backwards support for @c("stringId"). Remove before enabling in
-        // production so we accept only the identifier format.
-        lookahead(1, [&](CancellableBacktrackingScope &) {
-           return Tok.isNot(tok::string_literal);
-        })) {
-
+    if (AttrName == "c") {
       std::optional<StringRef> CName;
       if (consumeIfAttributeLParen()) {
         // Custom C name.

test/attr/attr_cdecl_official.swift

@@ -12,6 +12,10 @@
 // expected-error @-1 {{expected ')' in 'c' attribute}}
 // expected-error @-2 {{expected declaration}}
 
+@c("old string syntax") func stringSyntax() {}
+// expected-error @-1 {{expected C identifier in 'c' attribute}}
+// expected-error @-2 {{expected declaration}}
+
 @c() func emptyParen() {}
 // expected-error @-1 {{expected C identifier in 'c' attribute}}
 // expected-error @-2 {{expected declaration}}