[libsanitizer] Introduce flag descriptions.
Extend ParseFlag to accept the |description| parameter, add dummy values for all existing flags.
As the flags are parsed their descriptions are stored in a global linked list.
The tool can later call __sanitizer::PrintFlagDescriptions() to dump all the flag names and their descriptions.
Add the 'help' flag and make ASan, TSan and MSan print the flags if 'help' is set to 1.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204339 91177308-0d34-0410-b5e6-96231b3b80d8