SI-7147 Diagnostic for unexplained assertion in presentation compiler.

We don't have a reproducible test for this, so the best we can
do is beef up the assertion to shine a little light on the problem.

src/compiler/scala/tools/nsc/symtab/SymbolLoaders.scala

@@ -97,8 +97,15 @@ abstract class SymbolLoaders {
     val clazz = enterClass(root, name, completer)
     val module = enterModule(root, name, completer)
     if (!clazz.isAnonymousClass) {
-      assert(clazz.companionModule == module, module)
-      assert(module.companionClass == clazz, clazz)
+      // Diagnostic for SI-7147
+      def msg: String = {
+        def symLocation(sym: Symbol) = if (sym == null) "null" else s"${clazz.fullLocationString} (from ${clazz.associatedFile})"
+        sm"""Inconsistent class/module symbol pair for `$name` loaded from ${symLocation(root)}.
+            |clazz = ${symLocation(clazz)}; clazz.companionModule = ${clazz.companionModule}
+            |module = ${symLocation(module)}; module.companionClass = ${module.companionClass}"""
+      }
+      assert(clazz.companionModule == module, msg)
+      assert(module.companionClass == clazz, msg)
     }
   }