MC: Cleanup MCSectionMachO::ParseSectionSpecifier

Split by comma once instead of multiple times.  Moving this upfront
makes the rest of the code considerably simpler.

No functional change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203429 91177308-0d34-0410-b5e6-96231b3b80d8