Currently the symbol for MSPropertyDecl has kind `SymbolKind::Unknown`
which can trip up various indexing tools.
rdar://problem/46764224
Reviewers: akyrtzi, benlangmuir, jkorous
Reviewed By: jkorous
Subscribers: dexonsmith, cfe-commits, jkorous, jdoerfert, arphaman
Differential Revision: https://reviews.llvm.org/D57628
llvm-svn: 354942