[cxx-interop] Import operators renamed via swift_name consistently

Normally Swift imports `T& operator*` as `var pointee: T`. However, when the C++ operator has `swift_name` attribute attached to it, the importing was inconsistent: the renamed function was marked as deprecated, and an additional undesired overload was created.

For instance:
```
const std::string& operator*() const SWIFT_NAME(dereference());
```
was imported as an unavailable/deprecated `func dereference()`, and an additional overload of `func *(_:)` was created.

rdar://163500978

Author: egorzhdan

lib/ClangImporter/ImportDecl.cpp

@@ -3727,6 +3727,11 @@ namespace {
                                clang::OverloadedOperatorKind cxxOperatorKind) {
       if (cxxOperatorKind == clang::OverloadedOperatorKind::OO_None)
         return true;
+      // If this operator was renamed via swift_name attribute, the imported
+      // Swift function already has the specified name. Do not apply any special
+      // handling to it.
+      if (importedName.hasCustomName())
+        return true;
 
       auto dc = func->getDeclContext();
       auto typeDecl = dc->getSelfNominalTypeDecl();

test/Interop/Cxx/operators/Inputs/module.modulemap

@@ -17,3 +17,8 @@ module NonMemberOutOfLine {
   header "non-member-out-of-line.h"
   requires cplusplus
 }
+
+module RenamedOperators {
+  header "renamed-operators.h"
+  requires cplusplus
+}

test/Interop/Cxx/operators/Inputs/renamed-operators.h

@@ -0,0 +1,16 @@
+struct HasRenamedOperatorStar {
+  int value;
+
+  const int &operator*() const __attribute__((swift_name("dereference()"))) {
+    return value;
+  }
+};
+
+struct HasRenamedOperatorPlusPlus {
+  int value;
+
+  HasRenamedOperatorPlusPlus &operator++() __attribute__((swift_name("plusPlus()"))) {
+    value++;
+    return *this;
+  }
+};

test/Interop/Cxx/operators/renamed-operators-module-interface.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-ide-test -print-module -module-to-print=RenamedOperators -I %S/Inputs -source-filename=x -cxx-interoperability-mode=upcoming-swift | %FileCheck %s
+
+// CHECK: struct HasRenamedOperatorStar {
+// CHECK-NOT: prefix static func * (lhs: HasRenamedOperatorStar)
+// CHECK:   func dereference() -> UnsafePointer<Int32>
+// CHECK: }
+
+// CHECK: struct HasRenamedOperatorPlusPlus {
+// CHECK-NOT: prefix static func ++ (lhs: HasRenamedOperatorPlusPlus)
+// CHECK:   mutating func plusPlus() -> UnsafeMutablePointer<HasRenamedOperatorPlusPlus>
+// CHECK: }

test/Interop/Cxx/operators/renamed-operators-typechecker.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -cxx-interoperability-mode=default
+
+import RenamedOperators
+
+let star = HasRenamedOperatorStar(value: 123)
+_ = *star // expected-error {{'*' is not a prefix unary operator}}
+
+let plusPlus = HasRenamedOperatorStar(value: 123)
+plusPlus++ // expected-error {{cannot find operator '++' in scope; did you mean '+= 1'?}}