[modules] Apply ODR merging for function scoped tags only in C++ mode.

In C mode, if we have a visible declaration but not a visible definition, a tag
defined in the declaration should be have a visible definition. In C++ we rely
on the ODR merging, whereas in C we cannot because each declaration of a
function gets its own set of declarations in its prototype scope.

Patch developed in collaboration with Richard Smith!

llvm-svn: 280984