Fix conflict value for metadata "Objective-C Garbage Collection" in the mix of...

Fix conflict value for metadata "Objective-C Garbage Collection" in the mix of swift and Objective-C bitcode

Summary:
The change is to fix conflict value for metadata "Objective-C Garbage Collection" in the mix of swift and Objective-C bitcode.
The purpose is to provide the support of LTO for swift and Objective-C mixed project.

Reviewers: rjmccall, ahatanak, steven_wu

Reviewed By: rjmccall, steven_wu

Subscribers: manmanren, mehdi_amini, hiraditya, dexonsmith, llvm-commits, jinlin

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71219