Fix invalid shift operator overload in Scalar

Summary: This also fixes an infinite recursion between lldb_private::operator>> () and Scalar::operator>>= ().

Reviewers: sagar, tberghammer, labath

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D16868

Patch by Marianne Mailhot-Sarrasin

llvm-svn: 260239