Add logging verbosity level flag to extract_ir.py (#278)

* Add logging verbosity level flag to extract_ir.py

This commit adds a logging verbosity level flag to extract_ir.py and the
appropriate plumbing/implementation in extract_ir_lib.py. This is
primarily motivated by these errors coming up quite often in non-trivial
builds (as it is fairly often there is some assembly or some flags don't
get passed around somewhere) and not providing a very high signal to
noise ratio, especially when used as a library against a bunch of
projects at once.

* Address reviewer feedback

- Don't change logging verbosity back to info to print the status
  message if the verbosity has been set lower.

* Address reviewer feedback

- Log subprocess output directly rather than having subprocess write
  directly to STDOUT/STDERR.

* Address reviewer feedback

- Switch to using subprocess.check_output

compiler_opt/tools/extract_ir.py

@@ -28,6 +28,11 @@
 
 In a local ThinLTO case, the compilation is assumedto have been performed
 specifying -Wl,--save-temps=import -Wl,--thinlto-emit-index-files
+
+To change the logging verbosity, pass an integer representing the desired
+verbosity to the --verbosity flag. Use 0 for all logs, status information,
+and detailed debug information, -1 for solely warnings, and -2 to not produce
+any output.
 """
 
 import json

compiler_opt/tools/extract_ir_lib.py

@@ -135,9 +135,10 @@ def _extract_clang_artifacts(self, llvm_objcopy_path: str, cmd_filter: str,
       return None
     os.makedirs(self.dest_dir(), exist_ok=True)
     try:
-      subprocess.run(
+      subprocess.check_output(
           self._get_extraction_cmd_command(llvm_objcopy_path, cmd_section_name),
-          check=True)
+          stderr=subprocess.STDOUT,
+          encoding='utf-8')
       if cmd_filter is not None or is_thinlto:
         with open(self.cmd_file(), encoding='utf-8') as f:
           lines = f.readlines()
@@ -153,13 +154,15 @@ def _extract_clang_artifacts(self, llvm_objcopy_path: str, cmd_filter: str,
           index_file = get_thinlto_index(cmdline, self.obj_base_dir())
           shutil.copy(index_file, self.thinlto_index_file())
 
-      subprocess.run(
+      subprocess.check_output(
           self._get_extraction_bc_command(llvm_objcopy_path,
                                           bitcode_section_name),
-          check=True)
+          stderr=subprocess.STDOUT,
+          encoding='utf-8')
     except subprocess.CalledProcessError as e:
       # This may happen if  .o file was build from asm (.S source).
       logging.warning('%s was not processed: %s', self.input_obj(), e)
+      logging.info(e.output)
       return None
     assert (os.path.exists(self.cmd_file()) and
             os.path.exists(self.bc_file()) and