[analyzer] MIGChecker: Take advantage of the mig_server_routine annotation.

r354530 has added a new function/block/message attribute "mig_server_routine"
that attracts compiler's attention to functions that need to follow the MIG
server routine convention with respect to deallocating out-of-line data that
was passed to them as an argument.

Teach the checker to identify MIG routines by looking at this attribute,
rather than by making heuristic-based guesses.

rdar://problem/35380337

Differential Revision: https://reviews.llvm.org/58366

llvm-svn: 354638