[MCJIT] Clean up RuntimeDyld's quirky object-ownership/modification scheme.

Previously, when loading an object file, RuntimeDyld (1) took ownership of the
ObjectFile instance (and associated MemoryBuffer), (2) potentially modified the
object in-place, and (3) returned an ObjectImage that managed ownership of the
now-modified object and provided some convenience methods. This scheme accreted
over several years as features were tacked on to RuntimeDyld, and was both
unintuitive and unsafe (See e.g. http://llvm.org/PR20722).

This patch fixes the issue by removing all ownership and in-place modification
of object files from RuntimeDyld. Existing behavior, including debugger
registration, is preserved.

Noteworthy changes include:

(1) ObjectFile instances are now passed to RuntimeDyld by const-ref.
(2) The ObjectImage and ObjectBuffer classes have been removed entirely, they
    existed to model ownership within RuntimeDyld, and so are no longer needed.
(3) RuntimeDyld::loadObject now returns an instance of a new class,
    RuntimeDyld::LoadedObjectInfo, which can be used to construct a modified
    object suitable for registration with the debugger, following the existing
    debugger registration scheme.
(4) The JITRegistrar class has been removed, and the GDBRegistrar class has been
    re-written as a JITEventListener.

This should fix http://llvm.org/PR20722 .



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222810 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/ExecutionEngine.cpp

@@ -16,8 +16,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ExecutionEngine/GenericValue.h"
-#include "llvm/ExecutionEngine/ObjectBuffer.h"
-#include "llvm/ExecutionEngine/ObjectCache.h"
+#include "llvm/ExecutionEngine/JITEventListener.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -43,17 +42,15 @@ using namespace llvm;
 STATISTIC(NumInitBytes, "Number of bytes of global vars initialized");
 STATISTIC(NumGlobals  , "Number of global vars initialized");
 
-// Pin the vtable to this file.
-void ObjectCache::anchor() {}
-void ObjectBuffer::anchor() {}
-void ObjectBufferStream::anchor() {}
-
 ExecutionEngine *(*ExecutionEngine::MCJITCtor)(
     std::unique_ptr<Module> M, std::string *ErrorStr,
     RTDyldMemoryManager *MCJMM, std::unique_ptr<TargetMachine> TM) = nullptr;
 ExecutionEngine *(*ExecutionEngine::InterpCtor)(std::unique_ptr<Module> M,
                                                 std::string *ErrorStr) =nullptr;
 
+// Anchor for the JITEventListener class.
+void JITEventListener::anchor() {}
+
 ExecutionEngine::ExecutionEngine(std::unique_ptr<Module> M)
   : EEState(*this),
     LazyFunctionCreator(nullptr) {