Similar to `FILECHECK_OPTS` for FileCheck, `LIT_OPTS` makes it easy to
adjust lit behavior when running the test suite via ninja. For
example:
```
$ LIT_OPTS='--time-tests -vv --filter=threadprivate' \
ninja check-clang-openmp
```
Reviewed By: probinson
Differential Revision: https://reviews.llvm.org/D64135
llvm-svn: 365313