Add CommonLinkage; currently tentative definitions

Add CommonLinkage; currently tentative definitions
are represented as "weak", but there are subtle differences
in some cases on Darwin, so we need both.  The intent
is that "common" will behave identically to "weak" unless
somebody changes their target to do something else.
No functional change as yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51118 91177308-0d34-0410-b5e6-96231b3b80d8