Fix a diagnoses-valid bug with using declarations

The following was found by a customer and is accepted by the other primary
C++ compilers, but fails to compile in Clang:

namespace sss {
double foo(int, double);
template <class T>
T foo(T); // note: target of using declaration
}  // namespace sss

namespace oad {
void foo();
}

namespace oad {
using ::sss::foo;
}

namespace sss {
using oad::foo; // note: using declaration
}

namespace sss {
double foo(int, double) { return 0; }
template <class T>
T foo(T t) { // error: declaration conflicts with target of using
  return t;
}
}  // namespace sss

I believe the issue is that MergeFunctionDecl() was calling
checkUsingShadowRedecl() but only considering a FunctionDecl as a
possible shadow and not FunctionTemplateDecl. The changes in this patch
largely mirror how variable declarations were being handled by also
catching FunctionTemplateDecl.