[index] Improve indexing support for MSPropertyDecl.

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