[analyzer][UninitializedObjectChecker] New flag to ignore guarded uninitialized fields

This patch is an implementation of the ideas discussed on the mailing list[1].

The idea is to somewhat heuristically guess whether the field that was confirmed
to be uninitialized is actually guarded with ifs, asserts, switch/cases and so
on. Since this is a syntactic check, it is very much prone to drastically
reduce the amount of reports the checker emits. The reports however that do not
get filtered out though have greater likelihood of them manifesting into actual
runtime errors.

[1] http://lists.llvm.org/pipermail/cfe-dev/2018-September/059255.html

Differential Revision: https://reviews.llvm.org/D51866

llvm-svn: 352959