[lld-macho] Add more TimeTraceScopes

I added just enough to allow us to see a top-level breakdown of time taken. This
is the result of loading the time-trace output into `chrome:://tracing`:

https://gist.githubusercontent.com/int3/236c723cbb4b6fa3b2d340bb6395c797/raw/ef5e8234f3fdf609bf93b50f54f4e0d9bd439403/tracing.png

Reviewed By: oontvoo

Differential Revision: https://reviews.llvm.org/D99311