Some additions to tryEvaluate I've had sitting around for a while.

This pushes it a lot closer to being able to deal with most of the stuff 
CodeGen's constant expression evaluator knows how to deal with.  This 
also fixes PR3003.

The test could possibly use some improvement, but this'll work for now.  
Test 6 is inspired by PR3003; the other tests are mostly just designed
to exercise the new code.  The reason for the funny structure of the 
tests is that type fixing for arrays inside of structs is the only place 
in Sema that calls tryEvaluate, at least for the moment.

llvm-svn: 59125