tools: avoid use of std::function

tools: avoid use of std::function

Remove the use of the std::function and replace the capturing lambda with a
non-capturing one, opting to pass the user data down to the context.  This is
needed as std::function is not yet available on all hosted platforms (it
requires RTTI, which breaks on Windows).

Thanks to Nico Rieck for pointing this out!

llvm-svn: 209607