[lldb][progress] Mitigate non-specific LLDB progress reports in Swift

When LLDB reports progress on importing Swift modules, it was delivering
non-specific progress reports with only the title of "Importing Swift
modules" and no details. To report progress on activity within Swift,
LLDB first creates a progress report then updates that progress report
using a callback that LLDB sets and the Swift compiler invokes when
performing a full import.

When LLDB triggers Swift to import modules that were already imported before,
Swift will not perform a full import. Since the progress report would've
already been displayed, but the callback is never invoked, this leads
to the non-specific messages that were being displayed when importing
Swift modules.

This commit sets a unique pointer to create a progress report from
within the callback function instead of creating the report before
setting the callback. It also clears the callback on scope exit instead
of setting it to a new progress report.

Author: chelcassanova

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -2162,9 +2162,7 @@ SwiftASTContext::CreateInstance(lldb::LanguageType language, Module &module,
     auto on_exit = llvm::make_scope_exit([&]() {
       swift_ast_sp->m_ast_context_ap->SetPreModuleImportCallback(
           [](llvm::StringRef module_name,
-             swift::ASTContext::ModuleImportKind kind) {
-            Progress("Importing Swift modules");
-          });
+             swift::ASTContext::ModuleImportKind kind) {});
     });
 
     swift::ModuleDecl *stdlib =
@@ -2718,9 +2716,7 @@ lldb::TypeSystemSP SwiftASTContext::CreateInstance(
     auto on_exit = llvm::make_scope_exit([&]() {
       swift_ast_sp->m_ast_context_ap->SetPreModuleImportCallback(
           [](llvm::StringRef module_name,
-             swift::ASTContext::ModuleImportKind kind) {
-            Progress("Importing Swift modules");
-          });
+             swift::ASTContext::ModuleImportKind kind) {});
     });
 
     swift::ModuleDecl *stdlib =
@@ -3781,20 +3777,22 @@ swift::ModuleDecl *SwiftASTContext::GetModule(const SourceModule &module,
 
   // Report progress on module importing by using a callback function in
   // swift::ASTContext.
-  Progress progress("Importing Swift modules");
+  std::unique_ptr<Progress> progress;
   ast->SetPreModuleImportCallback(
       [&progress](llvm::StringRef module_name,
                   swift::ASTContext::ModuleImportKind kind) {
+        if (!progress)
+          progress = std::make_unique<Progress>("Importing Swift modules");
         switch (kind) {
         case swift::ASTContext::Module:
-          progress.Increment(1, module_name.str());
+          progress->Increment(1, module_name.str());
           break;
         case swift::ASTContext::Overlay:
-          progress.Increment(1, module_name.str() + " (overlay)");
+          progress->Increment(1, module_name.str() + " (overlay)");
           break;
         case swift::ASTContext::BridgingHeader:
-          progress.Increment(1,
-                             "Compiling bridging header: " + module_name.str());
+          progress->Increment(1, "Compiling bridging header: " +
+                                     module_name.str());
           break;
         }
       });
@@ -3804,9 +3802,7 @@ swift::ModuleDecl *SwiftASTContext::GetModule(const SourceModule &module,
   auto on_exit = llvm::make_scope_exit([&]() {
     ast->SetPreModuleImportCallback(
         [](llvm::StringRef module_name,
-           swift::ASTContext::ModuleImportKind kind) {
-          Progress("Importing Swift modules");
-        });
+           swift::ASTContext::ModuleImportKind kind) {});
   });
 
   // Perform the import.