[NFC] Add accessor for @_cdecl name

It also reads the C name out of imported C functions, which will be convenient for @cdecl @implementation.

Author: beccadax

include/swift/AST/Decl.h

@@ -2778,6 +2778,10 @@ class ValueDecl : public Decl {
     return DeclNameRef(Name);
   }
 
+  /// Retrieve the C declaration name that names this function, or empty
+  /// string if it has none.
+  StringRef getCDeclName() const;
+
   /// Retrieve the name to use for this declaration when interoperating
   /// with the Objective-C runtime.
   ///

lib/AST/Decl.cpp

@@ -3869,6 +3869,21 @@ void ValueDecl::setInterfaceType(Type type) {
                                         std::move(type));
 }
 
+StringRef ValueDecl::getCDeclName() const {
+  // Treat imported C functions as implicitly @_cdecl.
+  if (auto clangDecl = dyn_cast_or_null<clang::FunctionDecl>(getClangDecl())) {
+    if (clangDecl->getLanguageLinkage() == clang::CLanguageLinkage
+          && clangDecl->getIdentifier())
+      return clangDecl->getName();
+  }
+
+  // Handle explicit cdecl attributes.
+  if (auto *cdecl = getAttrs().getAttribute<CDeclAttr>())
+    return cdecl->Name;
+
+  return "";
+}
+
 llvm::Optional<ObjCSelector>
 ValueDecl::getObjCRuntimeName(bool skipIsObjCResolution) const {
   if (auto func = dyn_cast<AbstractFunctionDecl>(this))

lib/AST/SwiftNameTranslation.cpp

@@ -139,7 +139,7 @@ getObjCNameForSwiftDecl(const ValueDecl *VD, DeclName PreferredName){
 bool swift::objc_translation::
 isVisibleToObjC(const ValueDecl *VD, AccessLevel minRequiredAccess,
                 bool checkParent) {
-  if (!(VD->isObjC() || VD->getAttrs().hasAttribute<CDeclAttr>()))
+  if (!(VD->isObjC() || !VD->getCDeclName().empty()))
     return false;
   if (VD->getFormalAccess() >= minRequiredAccess) {
     return true;

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -1414,8 +1414,7 @@ class DeclAndTypePrinter::Implementation
 
     assert(FD->getAttrs().hasAttribute<CDeclAttr>() && "not a cdecl function");
     os << "SWIFT_EXTERN ";
-    printFunctionDeclAsCFunctionDecl(
-        FD, FD->getAttrs().getAttribute<CDeclAttr>()->Name, resultTy);
+    printFunctionDeclAsCFunctionDecl(FD, FD->getCDeclName(), resultTy);
     printFunctionClangAttributes(FD, funcTy);
     printAvailability(FD);
     os << ";\n";
@@ -1425,12 +1424,12 @@ class DeclAndTypePrinter::Implementation
     FunctionSwiftABIInformation(AbstractFunctionDecl *FD,
                                 LoweredFunctionSignature signature)
         : signature(signature) {
-      isCDecl = FD->getAttrs().hasAttribute<CDeclAttr>();
+      isCDecl = !FD->getCDeclName().empty();
       if (!isCDecl) {
         auto mangledName = SILDeclRef(FD).mangle();
         symbolName = FD->getASTContext().AllocateCopy(mangledName);
       } else {
-        symbolName = FD->getAttrs().getAttribute<CDeclAttr>()->Name;
+        symbolName = FD->getCDeclName();
       }
     }
 

lib/Sema/TypeCheckAttr.cpp

@@ -2118,7 +2118,7 @@ void AttributeChecker::visitExposeAttr(ExposeAttr *attr) {
   case ExposureKind::Cxx: {
     auto *VD = cast<ValueDecl>(D);
     // Expose cannot be mixed with '@_cdecl' declarations.
-    if (VD->getAttrs().hasAttribute<CDeclAttr>())
+    if (!VD->getCDeclName().empty())
       diagnose(attr->getLocation(), diag::expose_only_non_other_attr, "@_cdecl");
 
     // Nested exposed declarations are expected to be inside