[Parse] Fix empty platform condition crash

Handle the empty argument case in `getSingleSubExp`,
otherwise we'd crash with an index out of bounds.

rdar://83017601

Author: hamishknight

lib/Parse/ParseIfConfig.cpp

@@ -116,6 +116,9 @@ static llvm::VersionTuple getCanImportVersion(ArgumentList *args,
 
 static Expr *getSingleSubExp(ArgumentList *args, StringRef kindName,
                              DiagnosticEngine *D) {
+  if (args->empty())
+    return nullptr;
+
   if (auto *unary = args->getUnlabeledUnaryExpr())
     return unary;
 

test/Parse/ConditionalCompilation/basicParseErrors.swift

@@ -168,3 +168,8 @@ undefinedFunc() // expected-error {{cannot find 'undefinedFunc' in scope}}
 #else
 if true {}
 #endif // OK
+
+// rdar://83017601 Make sure we don't crash
+#if canImport()
+// expected-error@-1 {{expected only one argument to platform condition}}
+#endif