[PATCH] [libcxx] Add numerous options to libc++ LIT test suite configuration.

[Marshall Clow]

General comments:
1) I'm liking how this is going. A lot.
2) The symlink thing in using lit seems to be misguided to me. You should be able to specify a lit.site.cfg file on the command line. This will make it much easier to support multiple test configurations. I can think of 32 right off the bat: c++03/11/14/1z vs. 32/64 bit vs. No Sanitizer/ASAN/TSAN/UBSAN

3) Did I mention that I like the fast that it's much faster than testit? :-)

http://reviews.llvm.org/D5877