Fix cling tear down crash involving the ExternalSemaSource.

With the unique_ptr, we were crash due to the held over reference described
below.  Trying to move the m_ExternalSemaSource deletion after the m_IncrParser
deletion also leads to crash due to the double deletion (once by the ASTContext,
once by m_ExternalSemaSource) of the ExternalSemaSource.

If m_Callbacks is declared after m_IncrParser, it will be deleted first
and if m_ExternalSemaSource is a unique_ptr, it willbe deleted before
m_IncrParser is delete.

During the Incremental parser deletion, there is a call to
CodeGenModule::Release, then EmitTargetMetdata is called and it tries to
get the most recent decl which calls
     clang::LazyGenerationalUpdatePtr<...>::get
which has a LazyVal which has a cached pointer to the AST external
source.  So this placement is necessary to solve a tear down crash.

#0  clang::LazyGenerationalUpdatePtr<clang::Decl const*, clang::Decl*, &(clang::ExternalASTSource::CompleteRedeclChain(clang::Decl const*))>::get (this=0x7fff5fbfe0e0, O=0x1069c3f90) at ExternalASTSource.h:428
428	        (LazyVal->ExternalSource->*Update)(O);
(gdb) l
423	  /// Get the value of this pointer, updating its owner if necessary.
424	  T get(Owner O) {
425	    if (LazyData *LazyVal = Value.template dyn_cast<LazyData*>()) {
426	      if (LazyVal->LastGeneration != LazyVal->ExternalSource->getGeneration()) {
427	        LazyVal->LastGeneration = LazyVal->ExternalSource->getGeneration();
428	        (LazyVal->ExternalSource->*Update)(O);
429	      }
430	      return LazyVal->LastValue;
431	    }
432	    return Value.template get<T>();

#0  clang::LazyGenerationalUpdatePtr<clang::Decl const*, clang::Decl*, &(clang::ExternalASTSource::CompleteRedeclChain(clang::Decl const*))>::get (this=0x7fff5fbfe0e0, O=0x1069c3f90) at ExternalASTSource.h:428
#1  0x000000010005ce38 in clang::Redeclarable<clang::FunctionDecl>::DeclLink::getNext (this=0x1069c3ff0, D=0x1069c3f90) at Redeclarable.h:74
#2  0x000000010005cd81 in clang::Redeclarable<clang::FunctionDecl>::getNextRedeclaration (this=0x1069c3ff0) at Redeclarable.h:119
#3  0x000000010005d0b3 in clang::Redeclarable<clang::FunctionDecl>::getMostRecentDecl (this=0x1069c3ff0) at Redeclarable.h:161
#4  0x000000010137bfdc in clang::FunctionDecl::getMostRecentDeclImpl (this=0x1069c3f90) at Decl.h:1568
#5  0x00000001004b5346 in clang::Decl::getMostRecentDecl (this=0x1069c3f90) at DeclBase.h:806
#6  0x0000000100897128 in clang::CodeGen::CodeGenModule::EmitTargetMetadata (this=0x10689aa00) at /Users/pcanal/root_working/code/rootcling/interpreter/llvm/src/tools/clang/lib/CodeGen/CodeGenModule.cpp:3271
#7  0x0000000100895237 in clang::CodeGen::CodeGenModule::Release (this=0x10689aa00) at /Users/pcanal/root_working/code/rootcling/interpreter/llvm/src/tools/clang/lib/CodeGen/CodeGenModule.cpp:395
#8  0x0000000100981b3c in clang::CodeGeneratorImpl::ReleaseModule (this=0x106506d70) at /Users/pcanal/root_working/code/rootcling/interpreter/llvm/src/tools/clang/lib/CodeGen/ModuleBuilder.cpp:82
#9  0x00000001001c3f88 in ~IncrementalParser (this=0x1065047e0) at /Users/pcanal/root_working/code/rootcling/interpreter/cling/lib/Interpreter/IncrementalParser.cpp:168
#10 0x00000001001c3f25 in ~IncrementalParser (this=0x1065047e0) at /Users/pcanal/root_working/code/rootcling/interpreter/cling/lib/Interpreter/IncrementalParser.cpp:166
#11 0x00000001001d8905 in std::__1::default_delete<cling::IncrementalParser>::operator() () at /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/c++/v1/memory:2426
#12 0x00000001001d8905 in ~unique_ptr [inlined] () at /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/c++/v1/memory:2625
#13 0x00000001001d8905 in ~Interpreter (this=0x7fff5fbff020) at memory:2593
#14 0x00000001001d8355 in ~Interpreter (this=0x7fff5fbff020) at /Users/pcanal/root_working/code/rootcling/interpreter/cling/lib/Interpreter/Interpreter.cpp:229
#15 0x00000001002b0318 in main (argc=1, argv=0x7fff5fbff740) at /Users/pcanal/root_working/code/rootcling/interpreter/cling/tools/driver/cling.cpp:97