Make <experimental/filesystem> explicitly require C++11.

Previously the <experimental/filesystem> didn't guard its
contents in any dialect. However, the implementation implicitly
requires at least C++11, and the tests have always been marked
unsupported in C++03. This patch puts a header guard around the
contents to avoid exposing them before C++11.

Additionally, it replaces all of the usages of _NOEXCEPT or
_LIBCPP_CONSTEXPR with the keyword directly, since we can
expect the compiler to implement those by now.

llvm-svn: 337884