Compilation timings produced by `reportTime` are incorrect.

`reportTime` is used for recording timings of individual phases of compilation.

When the `--times:file.csv` is used, it produces a csv file with the following header:
```
Name,StartTime,EndTime,Duration(s),Id,ParentId,RootId,fileName,project,qualifiedNameOfFile,userOpName,length,cache,cpuDelta(s),realDelta(s),gc0,gc1,gc2,outputDllFile,buildPhase
```

Every `reportTime` call starts an activity. The StartTime of that activity is the time of the `reportTime` call.
The EndTime of the activity is the time of the _next_ `reportTime` call.

This means that code like this:
https://github.com/dotnet/fsharp/blob/615475b87bacc6728cb6a80acfd185983bf3ce8b/src/Compiler/AbstractIL/ilwritepdb.fs#L323-L324

produces incorrect `EndTime` and `Duration` value:
```
PDB: Sorted 84715 methods,21-34-19.7634,21-34-20.3083,000.5449,00-9c3f77575c3b7a5ca12bf645a553cada-2400157aee4f53f6-00,00-9c3f77575c3b7a5ca12bf645a553cada-d46ddb4fcab9901d-00,9c3f77575c3b7a5ca12bf645a553cada,,,,,,,,,,,,,
```

This made me spend quite a while thinking that there is ome low-hanging fruit in how those method objects are being sorted - sadly (thankfully?) there is none, and it's just an artefact of the report.

It would be nice to improve the reporting here.

CC @T-Gro