cmake: Error instead of warning and dropping invalid LLVM_USE_SANITIZER

Currently, if you call cmake with a typo in an LLVM_USE_SANITIZER
value, there's a cmake warning and the build goes on with no
sanitizers at all. This isn't a good behaviour, since cmake warnings
are fairly easy to miss and the resulting behaviour is that it looks
like the build is sanitizer clean.

Upgrade these warnings to errors so misconfigurations are more
obvious.

llvm-svn: 246531