build: allow building a specific set of sanitizers

Introduce a new CMake option `COMPILER_RT_SANITIZERS_TO_BUILD` which takes
either a special token `all` (default) which will preserve the current behaviour
or a CMake list of sanitizers to build.  It will still perform the normal checks
if the sanitizer is requested.  It only permits a further means to exclude a
particular sanitizer.  This gives finer grained control than
`COMPILER_RT_BUILD_SANITIZERS` which only gives an all or nothing control.

llvm-svn: 279253