Fix rdar://8139785 "implement warning on dead expression in comma operator"

Fix rdar://8139785 "implement warning on dead expression in comma operator"

As a bonus, fix the warning for || and && operators; it was emitted even if one of the operands had side effects, e.g:

x || test_logical_foo1();

emitted a bogus "expression result unused" for 'x'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107274 91177308-0d34-0410-b5e6-96231b3b80d8