Start a page tracking which C++ defect reports have been implemented in Clang.

The page is generated from a text file listing DR numbers and implementation
status, plus a copy of the cwg_index.html from the WG21 website. Recipe:

  $ wget http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_index.html
  $ ./make_cxx_dr_status >! cxx_dr_status.html

The intent here is to go through all the DRs, add tests for each one, then mark
them as done once such tests are committed and passing. I've not linked to this
page from anywhere, since it doesn't contain any useful information yet.

llvm-svn: 181967