Report a correct end location for nameless parameters.

Ranges before:
void test(void (*)(int), int, float);
          ~~~~~~~~~~~~~  ~~~~ ~~~~~~

Ranges after:
void test(void (*)(int), int, float);
          ~~~~~~~~~~~~~  ~~~  ~~~~~

This does not change the actual location of the ParmVarDecl, it still
points to the location where the name would be. PR17970.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@200640 91177308-0d34-0410-b5e6-96231b3b80d8