* Fix DerivedType::dropAllTypeUses to not change the number of types in a

  type when it gets refined.  This allows us to hash on this crucial value.
* Fix several issues in TypeMap::RefineAbstractType that prevent it from
  handling hash values that change correctly.
* Define hashTypeStructure to not always return 0.  :)

This last part (which depends on the first two) speeds up gccld time on eon
from 3.78s to 2.75s with a release build (a 28% speedup!).  This resolves
PR474.

llvm-svn: 24372