[interop] NFC, add a method to print out identifier to clang syntax printer

Author: hyp

lib/PrintAsClang/ClangSyntaxPrinter.cpp

@@ -37,6 +37,12 @@ bool ClangSyntaxPrinter::isClangKeyword(Identifier name) {
   return ClangSyntaxPrinter::isClangKeyword(name.str());
 }
 
+void ClangSyntaxPrinter::printIdentifier(StringRef name) {
+  os << name;
+  if (ClangSyntaxPrinter::isClangKeyword(name))
+    os << '_';
+}
+
 /// Print a C++ namespace declaration with the give name and body.
 void ClangSyntaxPrinter::printNamespace(
     llvm::function_ref<void(raw_ostream &OS)> namePrinter,

lib/PrintAsClang/ClangSyntaxPrinter.h

@@ -33,6 +33,10 @@ class ClangSyntaxPrinter {
 public:
   ClangSyntaxPrinter(raw_ostream &os) : os(os) {}
 
+  /// Print a given identifier. If the identifer conflicts with a keyword, add a
+  /// trailing underscore.
+  void printIdentifier(StringRef name);
+
   /// Print a C++ namespace declaration with the give name and body.
   void
   printNamespace(llvm::function_ref<void(raw_ostream &OS)> namePrinter,

lib/PrintAsClang/PrintClangFunction.cpp

@@ -96,9 +96,8 @@ void DeclAndTypeClangFunctionPrinter::printFunctionDeclAsCFunctionDecl(
     typePrinter.visit(ty, optionalKind);
 
     if (!name.empty()) {
-      os << ' ' << name;
-      if (ClangSyntaxPrinter::isClangKeyword(name))
-        os << '_';
+      os << ' ';
+      ClangSyntaxPrinter(os).printIdentifier(name);
     }
   };
 

test/Interop/SwiftToC/functions/swift-function-argument-keyword-in-c.swift

@@ -0,0 +1,7 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -typecheck -module-name Functions -clang-header-expose-public-decls -emit-clang-header-path %t/functions.h
+// RUN: %FileCheck %s < %t/functions.h
+
+// CHECK: SWIFT_EXTERN void $s9Functions19testKeywordArgument8registerySi_tF(ptrdiff_t register_) SWIFT_NOEXCEPT SWIFT_CALL;
+
+func testKeywordArgument(register: Int) { }