MC: Add support for '?' flags in .section directives

Summary:
The '?' flag uses the last section group if the last had a section
group.  We treat combining an explicit section group and the '?' as a
hard error.

This fixes PR17198.

Reviewers: rafael, bkramer

Reviewed By: bkramer

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1686

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190768 91177308-0d34-0410-b5e6-96231b3b80d8