DebugInfo: Don't emit vbase 'containing types' for context chain limited types

Possible minor reduction in debug info & avoid some cases where creating
a context chain could lead to the type the context chain is being
created for, being created. (this is still possible with template
parameters - tests/fixes/improvements to follow)

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@188639 91177308-0d34-0410-b5e6-96231b3b80d8