[cxx-interop] Do not crash for void begin()

When importing C++ methods, Swift always assumes that methods named `begin()` and `end()` are unsafe, since these methods commonly return iterator types that are inherently unsafe in Swift.

Some additional logic in Sema tries to diagnose usages of `.begin()` and `.end()` from Swift and suggest safe alternatives. That logic had a null pointer dereference bug.

rdar://153814676 / resolves #82361

Author: egorzhdan

lib/Sema/CSDiagnostics.cpp

@@ -4133,11 +4133,11 @@ void MissingMemberFailure::diagnoseUnsafeCxxMethod(SourceLoc loc,
                                                           scratch);
     };
 
-    auto returnTypeStr = cast<FuncDecl>(found)
-                             ->getResultInterfaceType()
-                             ->getAnyNominal()
-                             ->getName()
-                             .str();
+    auto returnTy =
+        cast<FuncDecl>(found)->getResultInterfaceType()->getAnyNominal();
+    if (!returnTy)
+      continue;
+    auto returnTypeStr = returnTy->getName().str();
 
     auto methodClangLoc = cxxMethod->getLocation();
     auto methodSwiftLoc =

test/Interop/Cxx/class/invalid-unsafe-projection-errors.swift

@@ -23,6 +23,11 @@ struct M {
   StringLiteral stringLiteral() const { return StringLiteral{"M"}; }
 };
 
+struct HasNonIteratorBeginMethod {
+  void begin() const;
+  void end() const;
+};
+
 //--- test.swift
 
 import Test
@@ -48,3 +53,8 @@ public func test(x: M) {
   // CHECK-NOT: error: value of type 'M' has no member 'stringLiteral'
   x.stringLiteral()
 }
+
+public func test(_ x: HasNonIteratorBeginMethod) {
+  x.begin()
+  x.end()
+}