os.remove shouldn't fail, if file doesn't exist

Summary:
os.remove might throw an exception (of type OSError), if given file
doesn't exist. Catch the exception, and ignore it //iff// errno is
ENOENT. Rethrow the exception, if errno is not ENOENT.

Reviewers: emaste

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6362

llvm-svn: 229334