Explicitly track the condition variable within an "if" statement,

Explicitly track the condition variable within an "if" statement,
rather than burying it in a CXXConditionDeclExpr (that occassionally
hides behind implicit conversions). Similar changes for
switch, while, and do-while will follow, then the removal of
CXXConditionDeclExpr. This commit is the canary.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@89717 91177308-0d34-0410-b5e6-96231b3b80d8