SI-6815 untangle isStable and hasVolatileType

`Symbol::isStable` is now independent of `Symbol::hasVolatileType`,
so that we can allow stable identifiers that are volatile in ident patterns.

This split is validated by SI-6815 and the old logic in RefChecks,
which seems to assume this independence, and thus I don't think ever worked:

```
if (member.isStable && !otherTp.isVolatile) {
  if (memberTp.isVolatile)
    overrideError("has a volatile type; cannot override a member with non-volatile type")
```

Introduces `admitsTypeSelection` and `isStableIdentifierPattern` in treeInfo,
and uses them instead of duplicating that logic all over the place.

Since volatility only matters in the context of type application,
`isStableIdentifierPattern` is used to check patterns (resulting in `==` checks)
and imports.

1 files changed, 5 insertions, 0 deletions

diff --git a/test/files/neg/t6815.check b/test/files/neg/t6815.check
new file mode 100644
index 0000000000..fae3819be1
--- /dev/null
+++ b/test/files/neg/t6815.check
@@ -0,0 +1,5 @@
+t6815.scala:15: error: stable identifier required, but Test.this.u.emptyValDef found.
+ Note that value emptyValDef is not stable because its type, Test.u.ValDef, is volatile.
+    case _: u.emptyValDef.T => // and, unlike in pos/t6185.scala, we shouldn't allow this.
+              ^
+one error found