Debug Info: add an assertion that the context field of a global variable can not
be a DIType with identifier.

This makes sure that there is no need to use DIScopeRef for global variable's
context.

rdar://18958417


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222561 91177308-0d34-0410-b5e6-96231b3b80d8