Stop defining negative versions of some lit feature keywords:

zlib/nozlib, asan/not_asan, msan/not_msan, ubsan/not_ubsan.

We still have two other ways to express the absence of a feature.
First, we have the '!' operator to invert the sense of a keyword.  For
example, given a feature that depends on zlib being unavailable, its
test can say:
    REQUIRES: !zlib

Second, if a test doesn't play well with some features, such as
sanitizers, that test can say:
    UNSUPPORTED: asan, msan

The different ways of writing these exclusions both have the same
technical effect, but have different implications to the reader.

llvm-svn: 360603