Revamp the "ConstantStruct::get" methods. Previously, these were scattered

all over the place in different styles and variants.  Standardize on two
preferred entrypoints: one that takes a StructType and ArrayRef, and one that
takes StructType and varargs.

In cases where there isn't a struct type convenient, we now add a
ConstantStruct::getAnon method (whose name will make more sense after a few
more patches land).  

It would be "really really nice" if the ConstantStruct::get and 
ConstantVector::get methods didn't make temporary std::vectors.

llvm-svn: 133412