[Support] Fix handle and memory leak for processes that are not waited for

Execute's Data parameter is now optional, so we won't allocate memory
for it on Windows and we'll close the process handle.

The Unix code should probably do something similar to avoid accumulation
of zombie children that haven't been waited on.

Tested on Linux and Windows.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183906 91177308-0d34-0410-b5e6-96231b3b80d8