Temporary fix to work around SI-6149.

When compiling with -DshowSuppressedErrors=false, parts of error
messages that are expensive to compute (like Type.toString)
are suppressed. This improves performance (about 20% for a full
build of Delite) because in many cases the type errors are
never shown to the user.

Conflicts:

	src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala

Conflicts:

	src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala