Sema: Do not merge new decls with invalid, old decls

Sema::MergeFunctionDecl attempts merging two decls even if the old decl
is invalid.  This can lead to interesting circumstances where we
successfully merge the decls but the result makes no sense.

Take the following for example:

template <typename T>
int main(void);

int main(void);

Sema will not consider these to be overloads of the same name because
main can't be overloaded, which means that this must be a redeclaration.

In this case the templated decl is compatible with the non-templated
decl allowing the Sema::CheckFunctionDeclaration machinery to move on
and do bizarre things like setting the previous decl of a non-templated
decl to a templated decl!

The way I see it, we should just bail from MergeFunctionDecl if the old
decl is invalid.

This fixes PR16531.

llvm-svn: 185779