[UBSan] Introduce "ubsan_standalone" library (compiler-rt part).

Get rid of "libclang_rt.san" library that used to contain
sanitizer_common pieces required by UBSan if it's used in a standalone
mode. Instead, build two variants of UBSan runtime: "ubsan" and
"ubsan_standalone" (same for "ubsan_cxx" and "ubsan_standalone_cxx").

Later "ubsan" and "ubsan_cxx" libraries will go away, as they will
embedded it into corresponding ASan runtimes.

llvm-svn: 233011