[libcxx] Update the way the -std= flag is chosen by CMake and LibcxxTestFormat

Summary:
This patch does two things:
CMake Update:
  - Add compiler flag checks for -std=c++11 and -std=c++1y and remove check for -std=c++0x.
  - Add configuration option LIBCXX_ENABLE_CXX1Y to prevent/allow -std=c++1y from being chosen as the std version. LIBCXX_ENABLE_CXX1Y is set to OFF by default.
  - if LIBCXX_ENABLE_CXX1Y is enabled then set LIBCXX_STD_VERSION to c++1y and fail if the compiler does not support -std=c++1y
  - If c++1y is not enabled then use c++11 and fail if the compiler does not support c++11.

Lit Update:
  - Update lit.site.cfg.in to capture LIBCXX_STD_VERSION information as config.std.
  - Remove mentions of has_cxx0X configuration option.
  - Check for `--param std=X' passed to lit on the command line.
  - Choose the std for the tests either from command line parameter or (if it doesn't exist) the lit.site.cfg.



Reviewers: mclow.lists, danalbert

Reviewed By: danalbert

Subscribers: emaste, rnk, ajwong, danalbert, cfe-commits

Differential Revision: http://reviews.llvm.org/D4329