Use -enable-global-merge option to match llvm r210639. <rdar://problem/17803206>

llvm revision 210639 renamed the -global-merge backend option to
-enable-global-merge. This change simply updates clang to match that.

Patch by Steven Wu!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213993 91177308-0d34-0410-b5e6-96231b3b80d8