SI-8933 Disable static Symbol literal cache in traits

In Scala 2.8.2, an optimization was added to create a static
cache for Symbol literals (ie, the results of `Symbol.apply("foo"))`.
This saves the map lookup on the second pass through code.

This actually was broken somewhere during the Scala 2.10 series,
after the addition of an overloaded `apply` method to `Symbol`.

The cache synthesis code was made aware of the overload and brought
back to working condition recently, in #3149.

However, this has uncovered a latent bug when the Symbol literals are
defined with traits.

One of the enclosed tests failed with:

	  jvm > t8933b-run.log
	java.lang.IllegalAccessError: tried to access field MotherClass.symbol$1 from class MixinWithSymbol$class
	        at MixinWithSymbol$class.symbolFromTrait(A.scala:3)
	        at MotherClass.symbolFromTrait(Test.scala:1)

This commit simply disables the optimization if we are in a trait.
Alternative fixes might be: a) make the static Symbol cache field
public / b) "mixin" the static symbol cache. Neither of these
seem worth the effort and risk for an already fairly situational
optimization.

Here's how the optimization looks in a class:

	% cat sandbox/test.scala; qscalac sandbox/test.scala && echo ":javap C" | qscala;
	class C {
	  'a; 'b
	}
	Welcome to Scala version 2.11.5-20141106-145558-aa558dce6d (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_20).
	Type in expressions to have them evaluated.
	Type :help for more information.

	scala> :javap C
	  Size 722 bytes
	  MD5 checksum 6bb00189166917254e8d40499ee7c887
	  Compiled from "test.scala"
	public class C

	{
	  public static {};
	    descriptor: ()V
	    flags: ACC_PUBLIC, ACC_STATIC
	    Code:
	      stack=2, locals=0, args_size=0
	         0: getstatic     #16                 // Field scala/Symbol$.MODULE$:Lscala/Symbol$;
	         3: ldc           #18                 // String a
	         5: invokevirtual #22                 // Method scala/Symbol$.apply:(Ljava/lang/String;)Lscala/Symbol;
	         8: putstatic     #26                 // Field symbol$1:Lscala/Symbol;
	        11: getstatic     #16                 // Field scala/Symbol$.MODULE$:Lscala/Symbol$;
	        14: ldc           #28                 // String b
	        16: invokevirtual #22                 // Method scala/Symbol$.apply:(Ljava/lang/String;)Lscala/Symbol;
	        19: putstatic     #31                 // Field symbol$2:Lscala/Symbol;
	        22: return

	  public C();
	    descriptor: ()V
	    flags: ACC_PUBLIC
	    Code:
	      stack=1, locals=1, args_size=1
	         0: aload_0
	         1: invokespecial #34                 // Method java/lang/Object."<init>":()V
	         4: getstatic     #26                 // Field symbol$1:Lscala/Symbol;
	         7: pop
	         8: getstatic     #31                 // Field symbol$2:Lscala/Symbol;
	        11: pop
	        12: return
	}

fixup
SI-7974 Fix over-eager optimization of Symbol literals

A classic mistake of discarding a non-trivial qualifier.

We actually should have fixed this before merging #3149, as it
was raised in review, but I suppose we got too caught up in the
difficulty of resolving the right overload of `Symbol_apply` that we
forgot.
Merge pull request #4095 from retronym/ticket/8933

Fix problems in Symbol Literal static caching