Preserve the lexical order for global variables during llvm-link merge

The order of global variables is generated in the order of recursively materializing variables if the global variable has the attribute of hasLocalLinkage or hasLinkOnceLinkage during the module merging. In practice, it is often the exact reverse of source order. This new order may cause performance regression.

The change is to preserve the original lexical order for global variables.

Reviewed By: jdoerfert, dexonsmith

Differential Revision: https://reviews.llvm.org/D94202