unnecessary copies of ClangDocContext

[vinniefalco]

It looks like ClangDocContext is copied several times needlessly.

However, it could be the case that in at least one of those cases, the copy is necessary so each thread has its own version.

[vinniefalco]

I added a counter and it looks like there is only ever one instance even though I ran a lot of compile commands concurrently. I will refactor the program to use references instead of making needless copies.