Add a factory method to ConstantDataArray that allows to pass in the data as StringRef

This simplifies the case if we already have access to the raw data that we need to store in a ConstantDataArray.
The new factor method can also be reused for implementing the factory method that gets the data as ArrayRef.

Differential Revision: https://reviews.llvm.org/D47706

llvm-svn: 335028