[CodeGen] First check the kind and then the llvm::Function properties.

[CodeGen] First check the kind and then the llvm::Function properties.

This patch fixes valgrind reports from downstream consumers about conditional
jump over uninitialised memory.

The original report:

```[ RUN      ] ScopeReflectionTest.IsComplete
==987150== Conditional jump or move depends on uninitialised value(s)
==987150==    at 0x1E1128F: clang::CodeGen::CodeGenModule::SetLLVMFunctionAttributesForDefinition(clang::Decl const*, llvm::Function*) (CodeGenModule.cpp:2391)
==987150==    by 0x1E4F181: clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (CodeGenModule.cpp:5669)
==987150==    by 0x1E4A194: clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (CodeGenModule.cpp:3909)
==987150==    by 0x1E4A752: clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (CodeGenModule.cpp:3649)
==987150==    by 0x1E532F5: clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) [clone .part.0] (CodeGenModule.cpp:6563)
==987150==    by 0x1B0BEDD: (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) (ModuleBuilder.cpp:190)
==987150==    by 0x1AEA47B: clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (CodeGenAction.cpp:235)
==987150==    by 0x101B02F: clang::IncrementalASTConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (IncrementalParser.cpp:52)
==987150==    by 0x101ED93: clang::IncrementalParser::ParseOrWrapTopLevelDecl() (IncrementalParser.cpp:276)
==987150==    by 0x101FBBC: clang::IncrementalParser::Parse(llvm::StringRef) (IncrementalParser.cpp:342)
==987150==    by 0x100E104: clang::Interpreter::Parse(llvm::StringRef) (Interpreter.cpp:360)
==987150==    by 0xE734C0: Cpp::Interpreter::Parse(llvm::StringRef) (CppInterOpInterpreter.h:172)
==987150==  Uninitialised value was created by a heap allocation
==987150==    at 0x844BE63: operator new(unsigned long) (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==987150==    by 0x1B0C882: StartModule (ModuleBuilder.cpp:139)
==987150==    by 0x1B0C882: clang::CodeGenerator::StartModule(llvm::StringRef, llvm::LLVMContext&) (ModuleBuilder.cpp:360)
==987150==    by 0x101C4AF: clang::IncrementalParser::GenModule() (IncrementalParser.cpp:372)
==987150==    by 0x101FC0E: clang::IncrementalParser::Parse(llvm::StringRef) (IncrementalParser.cpp:362)
==987150==    by 0x100E104: clang::Interpreter::Parse(llvm::StringRef) (Interpreter.cpp:360)
==987150==    by 0x100E243: clang::Interpreter::create(std::unique_ptr<clang::CompilerInstance, std::default_delete<clang::CompilerInstance> >) (Interpreter.cpp:279)
==987150==    by 0xF2131A: compat::createClangInterpreter(std::vector<char const*, std::allocator<char const*> >&) (Compatibility.h:123)
==987150==    by 0xF22AB9: Cpp::Interpreter::Interpreter(int, char const* const*, char const*, std::vector<std::shared_ptr<clang::ModuleFileExtension>, std::allocator<std::shared_ptr<clang::ModuleFileExtension> > > const&, void*, bool) (CppInterOpInterpreter.h:146)
==987150==    by 0xF1827A: CreateInterpreter (CppInterOp.cpp:2494)
==987150==    by 0xECFA0E: TestUtils::GetAllTopLevelDecls(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<clang::Decl*, std::allocator<clang::Decl*> >&, bool) (Utils.cpp:23)
==987150==    by 0xE9CB85: ScopeReflectionTest_IsComplete_Test::TestBody() (ScopeReflectionTest.cpp:71)
==987150==    by 0xF0ED0C: void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (in /home/vvassilev/workspace/builds/scratch/cppyy/InterOp/build-with-clang-repl-release/unittests/CppInterOp/CppInterOpTests)
==987150==
```

Differential revision: https://reviews.llvm.org/D159339

(cherry picked from commit 92246a9be0ba47788ada9621bef58ce7819be526)