[LinkerWrapper] Support time tracing in the linker wrapper

Summary:
This patch adds the new `--wrapper-time-trace=` option to write a time
tracing JSON file indicating where time was spent in the linker wrapper.
We also reformat and group some of the existing code to make
constraining the scope easier for time tracing.  We use the `--wrapper`
prefix to set this apart from the time tracing that lld may use.