[modules] Teach clang how to merge typedef over anonymous structs in C mode.

In C mode clang fails to merge the textually included definition with the one imported from a module. The C lookup rules fail to find the imported definition because its linkage is internal in non C++ mode.

This patch reinstates some of the ODR merging rules for typedefs of anonymous tags for languages other than C++.

Patch by Raphael Isemann and me (D34510).

llvm-svn: 306964