Clear the IRBuilder's insertion point after emitting a function.

The function might get finalized, invalidating the IP.
However, in some cases this invalid IP may get saved and restored, accessing the invalid IP while doing so.

Example code path accessing an invalid IP:
 -> jl_cfunction_object (nested_compile=true, but doesn't change IP)
 -> gen_cfun_wrapper
 -> jl_compile_linfo (saves and restores invalid IP)

src/codegen.cpp

@@ -4718,6 +4718,8 @@ static std::unique_ptr<Module> emit_function(jl_lambda_info_t *lam, jl_llvm_func
         }
     }
 
+    builder.ClearInsertionPoint();
+
     // step 13, Apply LLVM level inlining
     for(std::vector<CallInst*>::iterator it = ctx.to_inline.begin(); it != ctx.to_inline.end(); ++it) {
         Function *inlinef = (*it)->getCalledFunction();