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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133412 91177308-0d34-0410-b5e6-96231b3b80d8