[analyzer] Model trivial copy/move assignment operators with a bind as well.

r175234 allowed the analyzer to model trivial copy/move constructors as
an aggregate bind. This commit extends that to trivial assignment
operators as well. Like the last commit, one of the motivating factors here
is not warning when the right-hand object is partially-initialized, which
can have legitimate uses.

<rdar://problem/13405162>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177220 91177308-0d34-0410-b5e6-96231b3b80d8