[Expression parser] Setting to enable use of ExternalASTMerger

This setting can be enabled like this at the target level:

(lldb) settings set target.experimental.use-modern-type-lookup true

This causes several new behaviors in the Clang expression parser:

- It completely disables use of ClangASTImporter.  None are created
  at all, and all users of it are now conditionalized on its
  presence.

- It instead constructs a per-expression ExternalASTMerger, which
  exists inside Clang and contains much of the type completion
  logic that hitherto lived in ExternalASTSource,
  ClangExpressionDeclMap, and ClangASTImporter.

- The expression parser uses this Merger as a backend for copying
  and completing types.

- It also constructs a persistent ExternalASTMerger which is
  connected to the Target's persistent AST context.

This is a major chunk of LLDB functionality moved into Clang.  It
can be tested in two ways:

1. For an individual debug session, enable the setting before
   running a target.

2. For the testsuite, change the option to be default-true.  This
   is done in Target.cpp's g_experimental_properties.  The
   testsuite is not yet clean with this, so I have not committed
   that switch.

I have filed a Bugzilla for extending the testsuite to allow
custom settings for all tests:
  https://bugs.llvm.org/show_bug.cgi?id=34771

I have also filed a Bugzilla for fixing the remaining testsuite
failures with this setting enabled:
  https://bugs.llvm.org/show_bug.cgi?id=34772

llvm-svn: 314458

1 files changed, 8 insertions, 0 deletions

diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangModulesDeclVendor.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangModulesDeclVendor.cpp
index bce0eaf6d57..b42ceb9afee 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangModulesDeclVendor.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangModulesDeclVendor.cpp
@@ -85,6 +85,7 @@ public:
   void ForEachMacro(const ModuleVector &modules,
                     std::function<bool(const std::string &)> handler) override;
 
+  clang::ExternalASTMerger::ImporterSource GetImporterSource() override;
 private:
   void
   ReportModuleExportsHelper(std::set<ClangModulesDeclVendor::ModuleID> &exports,
@@ -109,6 +110,7 @@ private:
   typedef std::set<ModuleID> ImportedModuleSet;
   ImportedModuleMap m_imported_modules;
   ImportedModuleSet m_user_imported_modules;
+  const clang::ExternalASTMerger::OriginMap m_origin_map;
 };
 } // anonymous namespace
 
@@ -548,6 +550,12 @@ ClangModulesDeclVendorImpl::DoGetModule(clang::ModuleIdPath path,
                                          is_inclusion_directive);
 }
 
+clang::ExternalASTMerger::ImporterSource
+ClangModulesDeclVendorImpl::GetImporterSource() {
+  return {m_compiler_instance->getASTContext(),
+          m_compiler_instance->getFileManager(), m_origin_map};
+}
+
 static const char *ModuleImportBufferName = "LLDBModulesMemoryBuffer";
 
 lldb_private::ClangModulesDeclVendor *