Improve compile time of variant.

In particular, improve the compile time of the overload set builder
that variant uses to determine which alternative to construct.

Instead of having the __overload type construct itself recursively,
this patch uses a flat construction for the overload set.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366033 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed