Make -Wuninitialized warn on pointer-to-member and comma operators.

`isTrackedVar` has been updated to also track records.
`DeclRefExpr`s appearing on the left side of a comma operator are
ignored, while those appearing on the right side are classified as
`Use`.

Patch by Enrico Pertoso.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231068 91177308-0d34-0410-b5e6-96231b3b80d8