[lldb] Fix compilation with gcc-6.5

This fixes (works around) two errors with gcc-6.5.
- in the RegisterContext_x86 files, gcc is unable to synthesize a
  default constructor -- it thinks it needs to initialize the virtual
  base class, even though said classes are abstract. I fix that by
  providing a dummy constructor.
- In ReproducerInstrumentationTest, it is not able to deduce that the
  TestingRegistry class is movable (it contains a map of unique
  pointers). I change the type from Optional<TestingRegistry> to
  unique_ptr<TestingRegistry), so that moving is not required
  (copying/moving a polymorphic type is not a very good idea in any
  case).

lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux.h

@@ -58,6 +58,12 @@ public:
   virtual llvm::Optional<MmapData> GetMmapData() { return llvm::None; }
 
 protected:
+  // NB: This constructor is here only because gcc<=6.5 requires a virtual base
+  // class initializer on abstract class (even though it is never used). It can
+  // be deleted once we move to gcc>=7.0.
+  NativeRegisterContextLinux(NativeThreadProtocol &thread)
+      : NativeRegisterContextRegisterInfo(thread, nullptr) {}
+
   lldb::ByteOrder GetByteOrder() const;
 
   virtual Status ReadRegisterRaw(uint32_t reg_index, RegisterValue &reg_value);

lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.cpp

@@ -292,6 +292,8 @@ NativeRegisterContextLinux_x86_64::NativeRegisterContextLinux_x86_64(
     const ArchSpec &target_arch, NativeThreadProtocol &native_thread)
     : NativeRegisterContextRegisterInfo(
           native_thread, CreateRegisterInfoInterface(target_arch)),
+      NativeRegisterContextLinux(native_thread),
+      NativeRegisterContextDBReg_x86(native_thread),
       m_xstate_type(XStateType::Invalid), m_ymm_set(), m_mpx_set(),
       m_reg_info(), m_gpr_x86_64() {
   // Set up data about ranges of valid registers.

lldb/source/Plugins/Process/Utility/NativeRegisterContextDBReg_x86.h

@@ -16,6 +16,12 @@ namespace lldb_private {
 class NativeRegisterContextDBReg_x86
     : public virtual NativeRegisterContextRegisterInfo {
 public:
+  // NB: This constructor is here only because gcc<=6.5 requires a virtual base
+  // class initializer on abstract class (even though it is never used). It can
+  // be deleted once we move to gcc>=7.0.
+  NativeRegisterContextDBReg_x86(NativeThreadProtocol &thread)
+      : NativeRegisterContextRegisterInfo(thread, nullptr) {}
+
   Status IsWatchpointHit(uint32_t wp_index, bool &is_hit) override;
 
   Status GetWatchpointHitIndex(uint32_t &wp_index,

lldb/unittests/Utility/ReproducerInstrumentationTest.cpp

@@ -50,7 +50,7 @@ public:
   TestingRegistry();
 };
 
-static llvm::Optional<TestingRegistry> g_registry;
+static std::unique_ptr<TestingRegistry> g_registry;
 static llvm::Optional<Serializer> g_serializer;
 static llvm::Optional<Deserializer> g_deserializer;
 
@@ -75,13 +75,13 @@ inline TestInstrumentationData GetTestInstrumentationData() {
 class TestInstrumentationDataRAII {
 public:
   TestInstrumentationDataRAII(llvm::raw_string_ostream &os) {
-    g_registry.emplace();
+    g_registry = std::make_unique<TestingRegistry>();
     g_serializer.emplace(os);
     g_deserializer.reset();
   }
 
   TestInstrumentationDataRAII(llvm::StringRef buffer) {
-    g_registry.emplace();
+    g_registry = std::make_unique<TestingRegistry>();
     g_serializer.reset();
     g_deserializer.emplace(buffer);
   }