[libsanitizer] Add descriptions for the common flags.
Use new(allocator_for_flags) instead of allocator_for_flags.Allocate()
Fix the description output format a bit.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204484 91177308-0d34-0410-b5e6-96231b3b80d8