Add ConstantDataVector::getRaw() to create a constant data vector from raw data.

This parallels ConstantDataArray::getRaw() and can be used with ConstantDataSequential::getRawDataValues() in the base class for both types.

Update BuildConstantData{Array,Vector} tests to test the getRaw API. Also removes its unused Module.

In passing, update some comments to include the support for half and bfloat. Update tests to include testing for bfloat.

Differential Revision: https://reviews.llvm.org/D98302