[Polly] [Tests] Update `lit.cfg` uses of `lit.util.capture` to `subprocess.check_output`

[bollu]

Summary

• lit.util.capture was removed in r306625.
• Replace lit.util.capture to subprocess.check_output as LLVM did.
• LLVM revision of this change: https://reviews.llvm.org/D35088

[bollu]

• [NFC] remove whitespace

[grosser]

LGTM.

polly/trunk/test/lit.cfg

 # -*clang- Python -*-
 
 import os
 import platform
 import re
+import subprocess
 
 import lit.formats
 import lit.util
 
 # Configuration file for the 'lit' test runner.
 
 # name: The name of this test suite.
 config.name = 'Polly'
if config.test_exec_root is None:
     # and I decided it was too much magic. We should solve this by just having
     # the .cfg files generated during the configuration step.
 
     llvm_config = lit.util.which('llvm-config', config.environment['PATH'])
     if not llvm_config:
         lit_config.fatal('No site specific configuration available!')
 
     # Get the source and object roots.
-    llvm_src_root = lit.util.capture(['llvm-config', '--src-root']).strip()
-    llvm_obj_root = lit.util.capture(['llvm-config', '--obj-root']).strip()
+    llvm_src_root = subprocess.check_output(['llvm-config', '--src-root']).decode("utf-8").strip()
+    llvm_obj_root = subprocess.check_output(['llvm-config', '--obj-root']).decode("utf-8").strip()
+
     polly_src_root = os.path.join(llvm_src_root, "tools", "polly")
     polly_obj_root = os.path.join(llvm_obj_root, "tools", "polly")
 
     # Validate that we got a tree which points to here, using the standard
     # tools/polly layout.
     this_src_root = os.path.dirname(config.test_source_root)
     if os.path.realpath(polly_src_root) != os.path.realpath(this_src_root):
         lit_config.fatal('No site specific configuration available!')