[Index] Correctly set symbol kind of IndirectFieldDecl

Summary: The kind has been 'unknown' before, now it is 'field'.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62573

llvm-svn: 361941