Unify the ExecutionContextScope computation in Materializer.

This is an NFC cleanup for Clang, and a bugfix for the Swift
branch. In swift-lldb one target may have multiple scratch
TypeSystems, so it is important to pick the one that belongs to the
current frame, rather than the one for the current target.

<rdar://problem/65001402>

lldb/source/Expression/Materializer.cpp

@@ -785,7 +785,9 @@ class EntityResultVariable : public Materializer::Entity {
 
       const lldb::addr_t load_addr = process_address + m_offset;
 
-      ExecutionContextScope *exe_scope = map.GetBestExecutionContextScope();
+      ExecutionContextScope *exe_scope = frame_sp.get();
+      if (!exe_scope)
+        exe_scope = map.GetBestExecutionContextScope();
 
       llvm::Optional<uint64_t> byte_size = m_type.GetByteSize(exe_scope);
       if (!byte_size) {
@@ -835,7 +837,9 @@ class EntityResultVariable : public Materializer::Entity {
                      lldb::addr_t frame_bottom, Status &err) override {
     err.Clear();
 
-    ExecutionContextScope *exe_scope = map.GetBestExecutionContextScope();
+    ExecutionContextScope *exe_scope = frame_sp.get();
+    if (!exe_scope)
+      exe_scope = map.GetBestExecutionContextScope();
 
     if (!exe_scope) {
       err.SetErrorString("Couldn't dematerialize a result variable: invalid "
@@ -1061,7 +1065,9 @@ class EntitySymbol : public Materializer::Entity {
 
     const Address sym_address = m_symbol.GetAddress();
 
-    ExecutionContextScope *exe_scope = map.GetBestExecutionContextScope();
+    ExecutionContextScope *exe_scope = frame_sp.get();
+    if (!exe_scope)
+      exe_scope = map.GetBestExecutionContextScope();
 
     lldb::TargetSP target_sp;
 
@@ -1342,7 +1348,6 @@ Materializer::DematerializerSP
 Materializer::Materialize(lldb::StackFrameSP &frame_sp, IRMemoryMap &map,
                           lldb::addr_t process_address, Status &error) {
   ExecutionContextScope *exe_scope = frame_sp.get();
-
   if (!exe_scope)
     exe_scope = map.GetBestExecutionContextScope();
 
@@ -1393,7 +1398,9 @@ void Materializer::Dematerializer::Dematerialize(Status &error,
   if (thread_sp)
     frame_sp = thread_sp->GetFrameWithStackID(m_stack_id);
 
-  ExecutionContextScope *exe_scope = m_map->GetBestExecutionContextScope();
+  ExecutionContextScope *exe_scope = frame_sp.get();
+  if (!exe_scope)
+    exe_scope = m_map->GetBestExecutionContextScope();
 
   if (!IsValid()) {
     error.SetErrorToGenericError();