Allow "overloadable" functions in C to be declared as variadic without

Allow "overloadable" functions in C to be declared as variadic without
any named parameters, e.g., this is accepted in C:

  void f(...) __attribute__((overloadable));

although this would be rejected:

  void f(...);

To do this, moved the checking of the "ellipsis without any named
arguments" condition from the parser into Sema (where it belongs anyway).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64902 91177308-0d34-0410-b5e6-96231b3b80d8