Implement cross-module declaration merging for tag declarations, so

that if two modules A and B both contain a declaration of a tag such
as

  struct X;

and those two modules are unrelated, the two declarations of X will be
merged into a single redeclaration chain.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147488 91177308-0d34-0410-b5e6-96231b3b80d8