Sema: An extern declaration can't be a redeclaration of a parameter

Sema: An extern declaration can't be a redeclaration of a parameter

In the following:
void f(int x) { extern int x; }

The second declaration of 'x' shouldn't be considered a redeclaration of
the parameter.

This is a different approach to r225780.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@225875 91177308-0d34-0410-b5e6-96231b3b80d8