SI-6440 Address regressions around MissingRequirementError

 Go back to using globalError to report when a stub's info is referenced,
 and only throw the MissingRequirementError when compilation really
 must abort due to having a StubTermSymbol in a place where a
 StubClassSymbol would have been a better choice.

 This situation arises when an entire package is missing from the
 classpath, as was the case in the reported bug.

 Adds `StoreReporterDirectTest`, which buffers messages issued
 during compilation for more structured interrogation. Use this
 in two test for manifests -- these tests were using a crude means
 of grepping compiler console output to focus on the relevant output,
 but this approach was insufficient with the new multi-line error
 message emitted as part of this change.

 Also used that base test class to add two new tests: one for
 the reported error (package missing), and another for a simpler
 error (class missing). The latter test shows how stub symbols
 allow code to compile if it doesn't the subset of signatures
 in some type that refer to a missing class.

 Gave the INFO/WARNING/ERROR members of Reporter sensible
 toString implementations; they inherit from Enumeration#Value
 in an unusual manner (why?) that means the built in toString of
 Enumeration printed `Severity@0`.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index 67b2d05224..06bf54c1f8 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -3103,12 +3103,18 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     )
   }
   trait StubSymbol extends Symbol {
-    override final def failIfStub() = fail(())
     protected def missingMessage: String
+
+    /** Fail the stub by throwing a [[scala.reflect.internal.MissingRequirementError]]. */
+    override final def failIfStub() = {MissingRequirementError.signal(missingMessage)} //
+
+    /** Fail the stub by reporting an error to the reporter, setting the IS_ERROR flag
+      * on this symbol, and returning the dummy value `alt`.
+      */
     private def fail[T](alt: T): T = {
       // Avoid issuing lots of redundant errors
       if (!hasFlag(IS_ERROR)) {
-        MissingRequirementError.signal(missingMessage)
+        globalError(missingMessage)
         if (settings.debug.value)
           (new Throwable).printStackTrace