Clean up the C++0x __has_feature tests. Specifically:

  - Don't publicize a C++0x feature through __has_feature if we aren't
    in C++0x mode (even if the feature is available only with a
    warning). 
  - "auto" is not implemented well enough for its __has_feature to be
    turned on.
  - Fix the test of C++0x __has_feature to actually test what we're
  trying to test. Searching for the substring "foo" when our options
  are "foo" and "no_foo" doesn't work :)

llvm-svn: 124291