[-fms-extensions] Permit 'override' in C++98 and 'sealed' as a synonym for 'final'

Summary: Some MS headers use these features.

Reviewers: rnk, rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1948

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@192936 91177308-0d34-0410-b5e6-96231b3b80d8