Use matching lld version and adjust tcb span tuple struct definitions

[scamille]

The hope is that building with clang-7 gets faster.

[scamille]

Hmm after doing some research, I think the main cause of ccache misses is that -ffile-prefix-map is not available < clang10, leading to agent-dependent generated code, resulting in cache misses.

There isn't really much that can be done to prevent that.