Introduce a NullLog class, which ignores all messages.

The purpose of this class is so that GetLogIfAllCategoriesSet
can always return an instance of some class, whether it be a real
logging class or a "null" class, which ignores messages.  Code
that is littered with if statements that only log if the pointer
is non-null can get very unwieldy very quickly, so this should
help code readability in such circumstances.

Since I'm in this code anyway, I'm also deleting the
PrintfWithFlags methods, as well as all the flags, since they
appear to be dead code that have been superceded by newer
mechanisms and all the flags are simply ignored.

llvm-svn: 236174