SI-8962 Fix regression with skolems in pattern translation

During the refactoring of error reporting in 258d95c7b15, the result
of `Context#reportError` was changed once we had switched to
using a `ThrowingReporter`, which is still the case in post
typer phase typechecking

This was enough to provoke a type error in the enclosed test.
Here's a diff of the typer log:

    %  scalac-hash 258d95~1 -Ytyper-debug sandbox/test.scala 2>&1 | tee sandbox/good.log
    %  scalac-hash 258d95 -Ytyper-debug sandbox/test.scala 2>&1 | tee sandbox/bad.log
    %  diff -U100 sandbox/{good,bad}.log | gist --filename=SI-8962-typer-log.diff
    https://gist.github.com/retronym/3ccbe7e0791447d272a6

The test `Context#reportError` happens to be used when deciding
whether the type checker should be lenient when it hits a type error.
In lenient mode (see `adaptMismatchedSkolems`), it `adapt` retries
with after slackening the expected type by replacing GADT and
existential skolems with wildcard types. This is to accomodate
known type-incorrect trees generated by the pattern matcher.

This commit restores the old semantics of `reportError`, which means
it is `false` when a `ThrowingReporter` is being used.

For those still reading, here's some more details.

The trees of the `run2` example from the enclosed test. Notice that
after typechecking, `expr` has a type containing GADT skolems. The
pattern matcher assumes that calling the case field accessor `expr`
on the temporary val `x2 : Let[A with A]` containing the scrutinee
will result in a compatible expression, however this it does not.
Perhaps the pattern matcher should generate casts, rather than
relying on the typer to turn a blind eye.

```
[[syntax trees at end of                     typer]] // t8962b.scala
...
    def run2[A](nc: Outer2[A,A]): Outer2[Inner2[A],A] = nc match {
      case (expr: Outer2[Inner2[?A2 with ?A1],?A2 with ?A1])Let2[A with A]((expr @ _{Outer2[Inner2[?A2 with ?A1],?A2 with ?A1]}){Outer2[Inner2[?A2 with ?A1],?A2 with ?A1]}){Let2[A with A]} =>
        (expr{Outer2[Inner2[?A2 with ?A1],?A2 with ?A1]}: Outer2[Inner2[A],A]){Outer2[Inner2[A],A]}
    }{Outer2[Inner2[A],A]}

[[syntax trees at end of                    patmat]] // t8962b.scala

    def run2[A](nc: Outer2[A,A]): Outer2[Inner2[A],A] = {
      case <synthetic> val x1: Outer2[A,A] = nc{Outer2[A,A]};
      case5(){
        if (x1.isInstanceOf[Let2[A with A]])
          {
            <synthetic> val x2: Let2[A with A] = (x1.asInstanceOf[Let2[A with A]]: Let2[A with A]){Let2[A with A]};
            {
              val expr: Outer2[Inner2[?A2 with ?A1],?A2 with ?A1] = x2.expr{<null>};
              matchEnd4{<null>}((expr{Outer2[Inner2[?A2 with ?A1],?A2 with ?A1]}: Outer2[Inner2[A],A]){Outer2[Inner2[A],A]}){<null>}
              ...
```

1 files changed, 31 insertions, 0 deletions

diff --git a/test/files/pos/t8962.scala b/test/files/pos/t8962.scala
new file mode 100644
index 0000000000..4331c154ba
--- /dev/null
+++ b/test/files/pos/t8962.scala
@@ -0,0 +1,31 @@
+package test.nestedcov
+
+sealed abstract class Outer[+A]
+case class Let[+A](expr: Outer[Inner[A]]) extends Outer[A]
+
+sealed abstract class Inner[+A]
+
+sealed abstract class Outer2[+A, +B]
+case class Let2[+A](expr: Outer2[Inner2[A], A]) extends Outer2[A, A]
+
+sealed abstract class Inner2[+A]
+
+sealed abstract class Outer3[+A, +B]
+case class Let3[+A](expr: Outer3[A, A]) extends Outer3[A, A]
+
+object NestedCov {
+  def run[A](nc: Outer[A]) = nc match {
+    case Let(expr) =>
+      expr : Outer[Inner[A]]
+  }
+
+  def run2[A](nc: Outer2[A, A]) = nc match {
+    case Let2(expr) =>
+      expr : Outer2[Inner2[A], A]
+  }
+
+  def run3[A](nc: Outer3[A, A]) = nc match {
+    case Let3(expr) =>
+      expr : Outer3[A, A]
+  }
+}