[Sanitizers] Unify the semantics and usage of "exitcode" runtime flag across all sanitizers.

Summary:
Merge "exitcode" flag from ASan, LSan, TSan and "exit_code" from MSan
into one entity. Additionally, make sure sanitizer_common now uses the
value of common_flags()->exitcode when dying on error, so that this
flag will automatically work for other sanitizers (UBSan and DFSan) as
well.

User-visible changes:
* "exit_code" MSan runtime flag is now deprecated. If explicitly
  specified, this flag will take precedence over "exitcode".
  The users are encouraged to migrate to the new version.
* __asan_set_error_exit_code() and __msan_set_exit_code() functions
  are removed. With few exceptions, we don't support changing runtime
  flags during program execution - we can't make them thread-safe.
  The users should use __sanitizer_set_death_callback()
  that would call _exit() with proper exit code instead.
* Plugin tools (LSan and UBSan) now inherit the exit code of the parent
  tool. In particular, this means that ASan would now crash the program
  with exit code "1" instead of "23" if it detects leaks.

Reviewers: kcc, eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12120

llvm-svn: 245734