loader: only add Clang header path for CGo

It should only be added at the point that it is needed, for example when
using libclang or using the built-in Clang. It isn't needed when running
an external tool.

cgo/cgo_test.go

@@ -63,7 +63,7 @@ func TestCGo(t *testing.T) {
 			}
 
 			// Process the AST with CGo.
-			cgoAST, _, _, _, _, cgoErrors := Process([]*ast.File{f}, "testdata", fset, cflags)
+			cgoAST, _, _, _, _, cgoErrors := Process([]*ast.File{f}, "testdata", fset, cflags, "")
 
 			// Check the AST for type errors.
 			var typecheckErrors []error