Fix bug in ConditionBRVisitor where for C++ (and not C) we were not ignoring

Fix bug in ConditionBRVisitor where for C++ (and not C) we were not ignoring
implicit pointer-to-boolean conversions in condition expressions.  This would
result in inconsistent diagnostic emission between C and C++.

A consequence of this is now ConditionBRVisitor and TrackConstraintBRVisitor may
emit redundant diagnostics, for example:

  "Assuming pointer value is null" (TrackConstraintBRVisitor)
  "Assuming 'p' is null" (ConditionBRVisitor)

We need to reconcile the two, and perhaps prefer one over the other in some
cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163372 91177308-0d34-0410-b5e6-96231b3b80d8