[PATCH] [libc++] Check the support of default function template args.

[Marshall Clow]

  This seems like a really bad idea to me.

  Many of the template arguments that you are (conditionally) removing are, in fact, enable_if expressions, which control whether or not these specializations get considered in overload resolution.

  I have not tried this, but I would expect lots of test case failures when this compile flag is enabled.

http://llvm-reviews.chandlerc.com/D2384