Tweak LookThroughStmt() in LiveVariables to properly look through...

Tweak LookThroughStmt() in LiveVariables to properly look through alternativing ParenExprs and OpaqueValueExprs.  Thanks to Anna and Argiris for iterating on this function.  My original patch embarssingly didn't even pass the Clang tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143797 91177308-0d34-0410-b5e6-96231b3b80d8