Optimize and simplify SourcePosition handling

[odersky]

SourcePosition shows up high in the allocation charts: ~130'000 for
typer/.scala, i.e. 10 per line. We already have cut down on
SourcePosition allocations by passing Positioned instead of SourcePosition
where it makes sense. This PR generalizes this aproach: A new type
SrcPos creates a SourcePosition on demand. it is implemented
by SourcePosition itself, as well as Positioned and Symbol.
Where possible we use SrcPos instead of SourcePosition, thereby
delaying the creation of source positions. For typer/.scala, this
cut down allocations from 130K to 9K.

[odersky]

I propose to get this in quickly. It's a significant simplification to what we had before.

Before, report methods took SourcePositions which are expensive to create. So we did various tricks to avoid creating them, for instance by using Positioned types that carried the source position and that were passed around instead. Nevertheless, we still create about 10 source positions per source line, which makes source position one of the most often allocated types.

Now, we have a new type SrcPos that is extended by SourcePosition, Positioned, and Symbol. SrcPos is a type that provides a SourcePosition on request. report methods take SrcPos arguments.

[liufengyun]

LGTM

One conceptual question: can we reuse Positioned to play the role of SrcPos?

[liufengyun]

test performance please

[dottybot]

performance test scheduled: 7 job(s) in queue, 1 running.

[dottybot]

performance test failed:

Please check http://lamppc37.epfl.ch:8000/pull-9561-08-14-14.54.out for more information

[odersky]

test performance please

[dottybot]

performance test scheduled: 8 job(s) in queue, 1 running.

[odersky]

@liufengyun Positioned is a supertype of Tree and untpd.Mod, so it is more specific than SrcPos.

[dottybot]

Performance test finished successfully:

Visit http://dotty-bench.epfl.ch/9561/ to see the changes.

Benchmarks is based on merging with master (17c5dc1)