Fix clang-tidy to not assume wrong source locations for defaulted members.

Followup to http://reviews.llvm.org/D8465, which would break a test in
clang-tidy. clang-tidy previously assumes that source locations of
defaulted/deleted members are (incorrectly) not including the '= ...' part.

Differential Revision: http://reviews.llvm.org/D8466

llvm-svn: 233032