Avoid IFT in Reporter#reportsErrorsFor

If we make this method take an implicit function rather than a normal one we get a build error with trace:
```
[error] ## Exception when compiling 9 sources to /Users/odersky/workspace/dotty/sbt-bridge/src/target/classes
[error] Type scala.ImplicitFunction1 not present
[error] sun.reflect.generics.factory.CoreReflectionFactory.makeNamedType(CoreReflectionFactory.java:117)
[error] sun.reflect.generics.visitor.Reifier.visitClassTypeSignature(Reifier.java:125)
[error] sun.reflect.generics.tree.ClassTypeSignature.accept(ClassTypeSignature.java:49)
[error] sun.reflect.generics.repository.ConstructorRepository.getParameterTypes(ConstructorRepository.java:94)
[error] java.lang.reflect.Executable.getGenericParameterTypes(Executable.java:283)
[error] java.lang.reflect.Method.getGenericParameterTypes(Method.java:283)
[error] sbt.internal.inc.ClassToAPI$.parameterTypes(ClassToAPI.scala:566)
[error] sbt.internal.inc.ClassToAPI$.methodToDef(ClassToAPI.scala:318)
[error] sbt.internal.inc.ClassToAPI$.$anonfun$structure$1(ClassToAPI.scala:182)
[error] sbt.internal.inc.ClassToAPI$.$anonfun$mergeMap$1(ClassToAPI.scala:400)
[error] scala.collection.TraversableLike.$anonfun$flatMap$1(TraversableLike.scala:240)
[error] scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
[error] scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
[error] scala.collection.mutable.WrappedArray.foreach(WrappedArray.scala:37)
[error] scala.collection.TraversableLike.flatMap(TraversableLike.scala:240)
[error] scala.collection.TraversableLike.flatMap$(TraversableLike.scala:237)
[error] scala.collection.AbstractTraversable.flatMap(Traversable.scala:104)
[error] sbt.internal.inc.ClassToAPI$.merge(ClassToAPI.scala:411)
[error] sbt.internal.inc.ClassToAPI$.mergeMap(ClassToAPI.scala:400)
[error] sbt.internal.inc.ClassToAPI$.structure(ClassToAPI.scala:182)
[error] sbt.internal.inc.ClassToAPI$.x$2$lzycompute$1(ClassToAPI.scala:133)
[error] sbt.internal.inc.ClassToAPI$.x$2$1(ClassToAPI.scala:133)
[error] sbt.internal.inc.ClassToAPI$.instance$lzycompute$1(ClassToAPI.scala:133)
[error] sbt.internal.inc.ClassToAPI$.instance$1(ClassToAPI.scala:133)
[error] sbt.internal.inc.ClassToAPI$.$anonfun$toDefinitions0$1(ClassToAPI.scala:140)
[error] xsbti.api.SafeLazyProxy$$anon$1.get(SafeLazyProxy.scala:26)
[error] xsbti.api.SafeLazy$Impl.get(SafeLazy.java:58)

```

Author: odersky

compiler/src/dotty/tools/dotc/reporting/Reporter.scala

@@ -252,9 +252,9 @@ abstract class Reporter extends interfaces.ReporterResult {
 
   /** Run `op` and return `true` if errors were reported by this reporter.
    */
-  def reportsErrorsFor(op: given Context => Unit) given Context : Boolean = {
+  def reportsErrorsFor(op: Context => Unit) given (ctx: Context): Boolean = {
     val initial = errorCount
-    op
+    op(ctx)
     errorCount > initial
   }
 

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -1138,7 +1138,7 @@ trait Checking {
 
   /** check that annotation `annot` is applicable to symbol `sym` */
   def checkAnnotApplicable(annot: Tree, sym: Symbol) given (ctx: Context): Boolean =
-    !ctx.reporter.reportsErrorsFor {
+    !ctx.reporter.reportsErrorsFor { implicit ctx =>
       val pos = annot.sourcePos
       if (annot.symbol == defn.MainAnnot) {
         if (!sym.isRealMethod)