[Support] Fix llvm::unique_function when building with GCC 4.9 by

[Support] Fix llvm::unique_function when building with GCC 4.9 by
introducing llvm::trivially_{copy,move}_constructible type traits.

This uses a completely portable implementation of these traits provided
by Richard Smith. You can see it on compiler explorer in all its glory:

  https://godbolt.org/g/QEDZjW

I have transcribed it, clang-formatted it, added some comments, and made
the tests fit into a unittest file.

I have also switched llvm::unique_function over to use these new, much
more portable traits. =D

Hopefully this will fix the build bot breakage from my prior commit.

llvm-svn: 336161