Improve "assignment to cast" diagnostic.

 - Strip off extra parens when looking for casts.
 - Change the location info to point at the cast (instead of the
   assignment).

For example, on
  
  int *b;
  #define a ((void*) b)
  void f0() {
    a = 10;
  }
  
we now emit:
  
  /tmp/t.c:4:3: error: assignment to cast is illegal, lvalue casts are not supported
    a = 10;
    ^ ~
  /tmp/t.c:2:12: note: instantiated from:
  #define a ((void*) b)
            ~^~~~~~~~~~
  
instead of:
  
  /tmp/t.c:4:5: error: expression is not assignable
    a = 10;
    ~ ^

llvm-svn: 69114