[Sanitizer] Refactor SuppressionContext class.

SuppressionContext is no longer a singleton, shared by all sanitizers,
but a regular class. Each of ASan, LSan, UBSan and TSan now have their
own SuppressionContext, which only parses suppressions specific to
that sanitizer.

"suppressions" flag is moved away from common flags into tool-specific
flags, so the user now may pass
  ASAN_OPTIONS=suppressions=asan_supp.txt LSAN_OPIONS=suppressions=lsan_supp.txt
in a single invocation.

llvm-svn: 230026