my build time impact of clang 5.0

[Dan Mack]

Another significant change in build times this week - not complaining,
just my observations on build times; same server doing buildworld during
the various phases of compiler changes over the last year or so FWIW:

|--------------+--------------+---------------+----------+-----------|
| Ver (svn-id) | World (mins) | Kernel (mins) | Relative | Comment   |
|--------------+--------------+---------------+----------+-----------|
|       292733 |           90 |            16 |      0.5 |           |
|       299948 |           89 |            16 |      0.5 |           |
|       322724 |          174 |            21 |      1.0 | clang 4.x |
|       323310 |          175 |            21 |      1.0 | clang 4.x |
|       323984 |          175 |            21 |      1.0 | clang 4.x |
|       324130 |          285 |            21 |      1.6 | clang 5.x |
|       324204 |          280 |            21 |      1.6 | clang 5.x |
|--------------+--------------+---------------+----------+-----------|

Dan