[utils] collect_and_build_with_pgo.py: Print shell commands

[kazu]

Summary

This patch teaches collect_and_build_with_pgo.py to print shell
commands on --dry-run like:

(cd /home/kazu/llvm-project/llvm/out/instrumented &&
  ninja check-llvm check-clang)

instead of messages like:

Running `['ninja', 'check-llvm', 'check-clang']` in
/home/kazu/llvm-project/llvm/out/instrumented

This way, we could use the python script to generate a shell script
and debug things in the shell script form before updating the python
script. As another use case, we could use the shell script as a
starting point when we wish to heavily customize the build process.
For example, we could reuse the same profile to build several versions
of the optimized compiler with different sets of compiler flags.

llvm/utils/collect_and_build_with_pgo.py

class Env:
 
     # Note that we don't allow capturing stdout/stderr. This works quite nicely
     # with dry_run.
     def run_command(self,
                     cmd,
                     cwd=None,
                     check=False,
                     silent_unless_error=False):
-        print(
-            'Running `%s` in %s' % (cmd, shlex.quote(cwd or os.getcwd())))
+        print('(cd %s && %s)' % (shlex.quote(cwd or os.getcwd()),
+                                 ' '.join([shlex.quote(arg) for arg in cmd])))
 
         if self.dry_run:
             return
 
         if silent_unless_error:
             stdout, stderr = subprocess.PIPE, subprocess.STDOUT
         else:
             stdout, stderr = None, None

 
 
 def _build_things_in(env, target_dir, what):
     cmd = env.get_make_command() + what
     env.run_command(cmd, cwd=target_dir, check=True)
 
 
 def _run_fresh_cmake(env, cmake, target_dir):
-    if not env.dry_run:
+    if env.dry_run:
+        print('rm -rf %s' % shlex.quote(target_dir))
+        print('mkdir -p %s' % shlex.quote(target_dir))
+    else:
         try:
             shutil.rmtree(target_dir)
         except FileNotFoundError:
             pass
 
         os.makedirs(target_dir, mode=0o755)
 
     cmake_args = cmake.to_args()