Change all self assignments X=X to (void)X, so that we can turn on a

Change all self assignments X=X to (void)X, so that we can turn on a
new gcc warning that complains on self-assignments and
self-initializations.  Fix one bug found by the warning, in which one
clang::OverloadCandidate constructor failed to initialize its
FunctionTemplate member.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122459 91177308-0d34-0410-b5e6-96231b3b80d8