[analyzer] ConditionBRVisitor: Fix HTML PathDiagnosticPopUpPieces

Summary:
A condition could be a multi-line expression where we create the highlight
in separated chunks. PathDiagnosticPopUpPiece is not made for that purpose,
it cannot be added to multiple lines because we have only one ending part
which contains all the notes. So that it cannot have multiple endings and
therefore this patch narrows down the ranges of the highlight to the given
interesting variable of the condition. It prevents HTML-breaking injections.

Reviewed By: NoQ

Differential Revision: https://reviews.llvm.org/D65663

llvm-svn: 368382