Sema::SectionInfo: Prune "default" for now. Defaulted function is unavailable on msc17.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205840 91177308-0d34-0410-b5e6-96231b3b80d8
Sema::SectionInfo() should have an empty body. Sorry for the breakage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205841 91177308-0d34-0410-b5e6-96231b3b80d8