[clang][dataflow] Fix bug in joining bool values.

Currently, the code assumes that all boolean-typed values are an instance of
`BoolValue` (or its subclasses). Yet, lvalues violate this assumption. This
patch drops the assumption and strengthens the check to confirm the shape of
both values being joined.

The patch also notes as FIXMES a number of problems discovered fixing this bug.

Differential Revision: https://reviews.llvm.org/D141709