Sema: avoid reuse of Exprs when synthesizing operator=

Summary:
Previously, Sema was reusing parts of the AST when synthesizing an assignment
operator, turning it into a AS-dag. This caused problems for the static
analyzer, which assumed an expression appears in the tree only once.

Here I make sure to always create a fresh Expr, when inserting something into
the AST, fixing PR16745 in the process.

Reviewers: doug.gregor

CC: cfe-commits, jordan_rose

Differential Revision: http://llvm-reviews.chandlerc.com/D1425