[lld][LayoutPass] Consolidate debug flags, removing "layout" flag.

DEBUG_TYPE is already defined at the beginning of this file. We don't
want to have two different debug flags for a single pass.

Reviewers: shankarke

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D845

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@182543 91177308-0d34-0410-b5e6-96231b3b80d8