add some timing diagnostics

pass --enable-timing-info to print a nice table like this:

```
                Name       Start         End    Duration     Percent
          Initialize      0.0000      0.0000      0.0000      0.0001
   Semantic Analysis      0.0000      0.0421      0.0420      0.2109
     Code Generation      0.0421      0.0620      0.0200      0.1003
    LLVM Emit Object      0.0620      0.1852      0.1231      0.6180
  Build Dependencies      0.1852      0.1974      0.0122      0.0615
           LLVM Link      0.1974      0.1993      0.0018      0.0093
         Generate .h      0.1993      0.1993      0.0000      0.0000
               Total      0.0000      0.1993      0.1993      1.0000
```
e0050af293Andrew Kelley committed on 4/25/2017, 4:29:25 PMparent4cc2ea1
7 files changedLine totals unavailable