Don't try to fold DeclRefExprs that point to ParmVarDecls. This had the...

Don't try to fold DeclRefExprs that point to ParmVarDecls. This had the side-effect of always folding the expression to the default argument of the parameter. For example:

void f(int a = 10) {
  return a;
}

would always return 10, regardless of the passed in argument.

This fixes another 600 test failures. We're now down to only 137 failures!

llvm-svn: 95262