[SwiftREPL] Runtime checks for exclusive access should trap.

<rdar://problem/33831489>

lit/SwiftREPL/ExclusivityREPL.test

@@ -0,0 +1,20 @@
+// Runtime checks for exclusive access should be enabled in the REPL.
+
+// RUN: %lldb --repl < %s 2>&1 | FileCheck %s
+// CHECK: modification requires exclusive access
+// CHECK: Execution interrupted
+
+class C {
+  var f = 12
+}
+
+let c = C()
+
+func doIt(_ p1: inout Int, _ p2: inout Int) { }
+
+let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: -2)
+let flagPtr = dlsym(RTLD_DEFAULT, "_swift_disableExclusivityChecking")
+let flagValue = flagPtr!.load(as: Bool.self)
+
+print("flagValue is \(flagValue)")
+doIt(&c.f, &c.f) // This should trap

source/Target/SwiftLanguageRuntime.cpp

@@ -3861,8 +3861,10 @@ void SwiftLanguageRuntime::WillStartExecutingUserExpression() {
   std::lock_guard<std::mutex> lock(m_active_user_expr_mutex);
   Log *log(GetLogIfAnyCategoriesSet(LIBLLDB_LOG_EXPRESSIONS));
 
-  if (m_active_user_expr_count == 0 &&
-      m_dynamic_exclusivity_flag_addr) {
+  bool in_repl = m_process->GetTarget().GetDebugger().REPLIsActive();
+
+  if (m_active_user_expr_count == 0 && m_dynamic_exclusivity_flag_addr &&
+      !in_repl) {
     // We're executing the first user expression. Toggle the flag.
     Status error;
     TypeSystem *type_system =
@@ -3924,8 +3926,10 @@ void SwiftLanguageRuntime::DidFinishExecutingUserExpression() {
     log->Printf("SwiftLanguageRuntime: finished user expression. "
                 "Number active: %u", m_active_user_expr_count);
 
-  if (m_active_user_expr_count == 0 &&
-      m_dynamic_exclusivity_flag_addr) {
+  bool in_repl = m_process->GetTarget().GetDebugger().REPLIsActive();
+
+  if (m_active_user_expr_count == 0 && m_dynamic_exclusivity_flag_addr &&
+      !in_repl) {
     Status error;
     TypeSystem *type_system =
       m_process->GetTarget().GetScratchTypeSystemForLanguage(