[CodeView] Mark this pointers as const.

This is for compatibility with MSVC, which also marks this pointers
as being const-qualified.

Fixes llvm.org/pr36526

Differential Revision: https://reviews.llvm.org/D54736

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347353 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed