Return true from UnifySection when emitting a diagnostic

Test Plan: I noticed this through code inspection.  The callers use the return value to remove the SectionAttr if a diagnostic is emitted, but I don't think the failure to do so is observable right now.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5438

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218265 91177308-0d34-0410-b5e6-96231b3b80d8