[cfi] Better handling of wild target address.

This change enables diagnostics when the target address for a CFI
check is out of bounds of any known library, or even not in the
limits of the address space. This happens when casting pointers to
uninitialized memory.

Ubsan code does not yet handle some of these situations correctly,
so it is still possible to see a segmentation fault instead of a
proper diagnostic message once in a while.

llvm-svn: 258879