[FileCheck] Parse command-line options from FILECHECK_OPTS

This feature makes it easy to tune FileCheck diagnostic output when
running the test suite via ninja, a bot, or an IDE.  For example:

```
$ FILECHECK_OPTS='-color -v -dump-input-on-failure' \
  LIT_FILTER='OpenMP/for_codegen.cpp' ninja check-clang \
  | less -R
```

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D53517

llvm-svn: 346272