[lit] Try using process pools by default again

Both pickling errors encountered on clang bots and Darwin compiler-rt
should now be fixed.

This has no impact on testing time on Linux, and on Windows goes from
88s to 63s for 'check'. The tests pass on Mac, but I haven't compared
execution time.

llvm-svn: 299775

utils/lit/lit/main.py

@@ -302,7 +302,7 @@ def main_with_tmp(builtinParameters):
         opts.numThreads = lit.util.detectCPUs()
 
     if opts.executionStrategy is None:
-        opts.executionStrategy = 'PROCESSES'
+        opts.executionStrategy = 'PROCESS_POOL'
 
     if opts.maxFailures == 0:
         parser.error("Setting --max-failures to 0 does not have any effect.")