[GlobalMerge] Only emit aliases for internal linkage variables for non-Mach-O

[GlobalMerge] Only emit aliases for internal linkage variables for non-Mach-O

On Mach-O emitting aliases for the variables that make up a MergedGlobals
variable can cause problems when linking with dead stripping enabled so don't
do that, except for external variables where we must emit an alias.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244748 91177308-0d34-0410-b5e6-96231b3b80d8