Execute llvm-lit with the python found by CMake by default

The check-* targets run ${Python3_EXECUTABLE} $BUILD/bin/llvm-lit, but
running `./bin/llvm-lit $ARGS` from the build directory currently always
uses "python" to run llvm-lit. On most systems this will be python2.7 even
if we found python3 at CMake time.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D84625

llvm/utils/gn/secondary/llvm/utils/llvm-lit/BUILD.gn

@@ -86,6 +86,7 @@ write_cmake_config("llvm-lit") {
 
   values = [
     "LLVM_SOURCE_DIR=" + rebase_path("//llvm", dir),
+    "Python3_EXECUTABLE=$python_path",
     "BUILD_MODE=.",
     "LLVM_LIT_CONFIG_MAP=" + config_map,
   ]

llvm/utils/llvm-lit/llvm-lit.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!@Python3_EXECUTABLE@
 # -*- coding: utf-8 -*-
 
 import os