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