Fix PR37407 - callable traits don't correctly check complete types.

Checking for complete types is really rather tricky when you consider
the amount of specializations required to check a function type. This
specifically caused PR37407 where we incorrectly diagnosed
noexcept function types as incomplete (but there were plenty of other
cases that would cause this).

This patch removes the complete type checking for now. I'm going
to look into adding a clang builtin to correctly do this for us.

llvm-svn: 332040