SI-7329 duplicate default getters for specialized parameters.

The default getter is generated with @specialized annotation if the
type parameter corresponding to the type of the parameter is
specialized. Consequently specialize pass tries to generate overloads.

Rather than pruning overloads to exclude duplicates, let's notice
that default getter specialization is not needed at all:
  - The dynamic scope of default getter doesn't include specialized
    method or class constructor.
  - generic default getter is called even when calling specialized
    method:
    object V {
      @specialized def foo[@specialized B](b: B = (??? : B)) = {}
      foo[Int]()
    }
    gives:
      invokevirtual Method V$.foo$default$1:()Ljava/lang/Object;
      invokestatic (unboxToInt)
      invokevirtual Method V$.foo$mIc$sp:(I)V
SI-7319 Clear error buffer during Typer reset.

Contexts share error/warning buffers with their
children, and this also applies ot the shared
`startContext`.

That context flushes the buffers in `startContext`
in `resetContexts`.

It also removes `typerReportAnyContextErrors`, which
appears to be an elaborate no-op. It is only ever passed
a context `c` which is a direct child of `this.context`.
So taking a buffered error out of `c` and reissuing it
into `this.context` merely re-inserts into into the same
error buffer. Consrast this with `silent`, which uses
a child context with a fresh error buffer.

SI-7319 Flush error buffer in typerReportAnyContextErrors.

After this change, we no longer rely on the backstop in resetContexts
introduced in the previous commit.
SI-7377 Fix retypechecking of patterns on case companion alias

Some ancient code in Typers switches from PATTERNmode to
EXPRmode when encountering `stableF(...)`. It just
typechecks `stableF` and discards the arguments.

To the best of Martin's recollection, this has something
to do with the need to typecheck patterns rather late in
the compiler, after `a.b` had been translated to `a.b()`
in `Uncurry`. I'm not able to motivate this with tests
using `-Xoldpatmat`; was there ever an even older pattern
matcher that ran *after* uncurry?

What changed in 2.10.1 to expose this wrinkle? dfbaaa17
fixed `TypeTree.copyAttrs` to copy the original tree.
During the descent of `ResetAttrs`, sub-trees are duplicated
before begin further transformed. Duplicating the `Match`
in 2.10.0 would forget that the original tree of:

    pat = (a: Int)Foo(_)
          `----------`
               `- TypeTree((a: Int)Foo), with original Select(..., "FooAlias")

The retypechecking would operate on the `MethodType`, rather
than the `Select`, which was not considered a stable
application.

For 2.10.x, I've just tightened up the condition to only
hit this if `args` is empty. I'm almost certain that the code
can be removed altogether, and I'll do that when this is merged
to master.
Absolute path in error message.

As soon as you have a directory called "language" lying around,
you will appreciate why the advice given regarding SIP-18
should be "import scala.language..." not "import language..."
SI-7388 Be more robust against cycles in error symbol creation.

`Symbol#toString` was triggering `CyclicReferenceError` (specifically,
`accurateKindString` which calls `owner.primaryConstructor`.)

The `toString` output is used when creating an error symbol to
assign to the tree after an error (in this case, a non-existent
access qualifier.)

This commit catches the error, and falls back to just using the
symbol's name.
Merge pull request #2364 from vigdorchik/ticket/si-7329

SI-7329 duplicate default getters for specialized parameters.
Merge pull request #2370 from retronym/ticket/7319-2

SI-7319 Avoid unflushed error/warning buffers in startContext
Merge pull request #2402 from retronym/ticket/7377

SI-7377 Fix retypechecking of patterns on case companion alias
Merge pull request #2411 from retronym/ticket/7388

SI-7388 Be more robust against cycles in error symbol creation.
Merge pull request #2408 from paulp/pr/fully-qualified-name

Absolute path in error message.
Merge remote-tracking branch 'origin/2.10.x' into merge/v2.10.1-245-g5147bb2-to-master

Conflicts:
	src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
	src/compiler/scala/tools/nsc/typechecker/Typers.scala
SI-7377 Remove special treatment of `stableFun()` in patterns.

As promised in 15e9ef8f08.
SI-7319 Remove unused method.

Detritus spottend after merging SI-7319 from 2.10.x to master.
Merge pull request #2419 from retronym/merge/v2.10.1-245-g5147bb2-to-master

Merge v2.10.1-245-g5147bb2 to master