builder: refactor clang include headers

Set -resource-dir in a central place instead of passing the header path
around everywhere and adding it using the `-I` flag. I believe this is
closer to how Clang is intended to be used.

This change was inspired by my attempt to add a Nix flake file to
TinyGo.

transform/transform_test.go

@@ -145,7 +145,7 @@ func compileGoFileForTesting(t *testing.T, filename string) llvm.Module {
 	defer machine.Dispose()
 
 	// Load entire program AST into memory.
-	lprogram, err := loader.Load(config, filename, config.ClangHeaders, types.Config{
+	lprogram, err := loader.Load(config, filename, types.Config{
 		Sizes: compiler.Sizes(machine),
 	})
 	if err != nil {