Fix range of coerce-expr when casting a returned CFString to String

    foo().getCFString() as String

This cast creates an implicit call expression where the function for the
call is an implicit declref. Instead of choosing "getCFString" as the
location of the declref, we should choose "foo", since otherwise the
overall source range for the call and the coercion will be missing
foo().

Unfortunately there is no good way to test source ranges right now, but
this fixes e.g. the range in the following diagnostic:

    struct S {
      func getCFString() -> CFString { fatalError("") }
    }

    _ = S().getCFString() as String + 1
            ~~~~~~~~~~~~~~~~~~~~~~~ ^ ~  // before
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~  // now

rdar://problem/26301228

Author: benlangmuir

lib/Sema/CSApply.cpp

@@ -1439,7 +1439,7 @@ namespace {
 
       ConcreteDeclRef fnSpecRef(tc.Context, fn, Subs);
       Expr *fnRef = new (tc.Context) DeclRefExpr(fnSpecRef,
-                                                 DeclNameLoc(object->getLoc()),
+                                                 DeclNameLoc(object->getStartLoc()),
                                                  /*Implicit=*/true);
       TypeSubstitutionMap subMap;
       auto genericParam = fnGenericParams[0];