[CodeGen][ObjC] Annotate retain-agnostic ObjC globals with attribute

[CodeGen][ObjC] Annotate retain-agnostic ObjC globals with attribute
'objc_arc_inert'

The attribute enables the ARC optimizer to delete ObjC ARC runtime calls
on the annotated globals (see https://reviews.llvm.org/D62433). We
currently only annotate global variables for string literals and global
blocks with the attribute.

rdar://problem/49839633

Differential Revision: https://reviews.llvm.org/D62831

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@363467 91177308-0d34-0410-b5e6-96231b3b80d8