[analyzer] Improve "Assuming..." diagnostic pieces for logical operators.

Logical short-circuit operators now act like other branch conditions.

If the symbolic value of the left-hand side is not known to be true or false
(based on the previous execution path), the "Assuming" event piece is added
in order to explain that the analyzer is adding a new assumption.

Additionally, when the assumption is made against the right-hand side of
the logical operator (i.e. when the operator itself acts as a condition
in another CFG terminator), the "Assuming..." piece is written out for the
right-hand side of the operator rather than for the whole operator.
This allows expression-specific diagnostic message text to be constructed.

Differential Revision: https://reviews.llvm.org/D25092

llvm-svn: 283302