SI-6022 refactor to clean up model of variable equality modulo <:

this commit does a functionality-preserving refactoring
SI-6022 cleaner model of variable equality modulo <:

a more conservative "excludes": no need to reason about types

(TODO: check we don't get any spurious unreachability errors
 in the eclipse build, which is a good canary for this kind of thing)
Support per-group flags files in partest

In a directory-based test with file A_1.scala, scalac flags that should apply only to A_1.scala can be put in the file A_1.flags. Local flags are appended to global flags.
Added make not private logic for @inline accesses back to ExplicitOuter.

As discussed in https://mail.google.com/mail/u/0/?hl=en&tab=Tm#search/inlining+changes+needed/1390c955960d7cb5, we need to move access widenings back to ExplicitOuter, so that outer fields are correctly widened. The previous logiv in SuperAccessors can go once the inliner is adapted to the new scheme. I leave SuperAccessors for the moment as is, in order not to slide back even further with inlining.
More resilience to missing classes.

The situation (I don't know how to make partest test this) is

  package s
  class A ; class S { def f(): A = ??? }

If one compiles this and removes A.class, should references to
class S cause the compiler to explode eagerly and fail to load S,
or explode lazily if and when it needs to know something about A?
This patch takes us from the former strategy to the latter.

Review by @xeno-by.
Fixes SI-6214.

addSynthetics was calling typedStat that caused flushing error buffer. Typically that would result in errors being re-added but addSynthetics was the last thing to run in typedStats.
Instead of adding yet another check I used this as an opportunity to cleanup the once necessary workaround for typing stats. Since we are now chaining buffers of contexts manual handling is no longer necessary.

Review by @odersky (I recall that you were planning to cleanup that part at some point).
Merge pull request #1123 from paulp/topic/unpickle-resilience-2

Topic/unpickle resilience 2
Merge pull request #1125 from hubertp/2.10.x-issue/6214

Fixes SI-6214.
Move side-effecting call out of debuglog(). Fixes SI-6223.

This change fixes a situation in which method calls aren't being properly
specialized. All the existing tests pass; in addition, it likely fixes other
specialization bugs (and improves performance of specialized code).

Also includes a small regression test.
Merge pull request #1121 from odersky/topic/inline2

Added make not private logic for @inline accesses back to ExplicitOuter.
Fix for view isEmpty.

Views have been inheriting the very inefficient isEmpty
from Traversable, and since isEmpty is not specifically
forwarded to the underlying collections, views miss out on all
the important optimizations in those collections which tend to
be implemented via method override.  Not to mention, they miss
out on correctness, because calling foreach has a habit of
forcing the first element of the view.
Merge pull request #1102 from phaller/partest-local-flags

Support per-group flags files in partest
Merge pull request #1100 from adriaanm/ticket-6022b

SI-6022 cleaner model of variable equality modulo <:
Merge pull request #1130 from non/si6223-2.10.x

Move side-effecting call out of debuglog(). Fixes SI-6223.
Merge pull request #1131 from paulp/topic/view-isempty

Topic/view isempty