Prevent position errors on Ident(nme.ERROR)

```
Exception in thread "main" java.lang.AssertionError: assertion failed: position error: position not set for Ident(<error>) # 24
	at dotty.DottyPredef$.assertFail(DottyPredef.scala:36)
	at dotty.tools.dotc.ast.Positioned.check$1(Positioned.scala:178)
	at dotty.tools.dotc.ast.Positioned.check$5$$anonfun$4(Positioned.scala:203)
```

Author: Blaisorblade

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -518,8 +518,16 @@ object Parsers {
       }
 
     /** Accept identifier and return Ident with its name as a term name. */
-    def termIdent(): Ident = atPos(in.offset) {
-      makeIdent(in.token, ident())
+    def termIdent(): Ident = {
+      val start = in.offset
+      val id = makeIdent(in.token, ident())
+      if (id.name != nme.ERROR) {
+        atPos(start)(id)
+      } else {
+        // error identifiers don't consume any characters, so atPos(start)(id) wouldn't set a position.
+        // Some testcases would then fail in Positioned.checkPos. Set a position anyway!
+        atPos(start, start, in.lastOffset)(id)
+      }
     }
 
     /** Accept identifier and return Ident with its name as a type name. */

tests/neg/parser-stability-23.scala

@@ -0,0 +1,3 @@
+object i0 {
+  import Ordering.{ implicitly => } (true: Boolean) match { case _: i1 => true } // error // error
+}