Change ConstantArray::get to form a ConstantDataArray when possible,

kicking in the big win of ConstantDataArray.  As part of this, change
the implementation of GetConstantStringInfo in ValueTracking to work
with ConstantDataArray (and not ConstantArray) making it dramatically,
amazingly, more efficient in the process and renaming it to 
getConstantStringInfo.

This keeps around a GetConstantStringInfo entrypoint that (grossly)
forwards to getConstantStringInfo and constructs the std::string 
required, but existing clients should move over to 
getConstantStringInfo instead.

llvm-svn: 149351