Add more information to the log timer dump

Summary:
The `log timer dump` is showing the time of the function itself minus any function that is called from this one that also happens to be timed. However, this is really not obvious and it also makes it hard to understand the time spent in total and also which children are actually taking the time.
To get a better reading of the timer dump I added the total, children (which I named child) and also the hit count. I used these timers to figure out a performance issue and only after adding this things were more clear to me.

It looks like this:
```
(lldb) log timer dump
35.447713617 sec (total: 35.449s; child: 0.001s; count: 1374) for void SymbolFileDWARF::Index()
29.717921481 sec (total: 29.718s; child: 0.000s; count: 8230500) for const lldb_private::ConstString &lldb_private::Mangled::GetDemangledName(lldb::LanguageType) const
21.049508865 sec (total: 24.683s; child: 3.633s; count: 1399) for void lldb_private::Symtab::InitNameIndexes()
...
```

Reviewers: clayborg, teemperor, labath, espindola, xiaobai

Reviewed By: labath, xiaobai

Subscribers: emaste, mgorny, arichardson, eraman, MaskRay, jdoerfert, labath, davide, teemperor, aprantl, erik.pilkington, jfb, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61235

llvm-svn: 361987