Convert attribute 'target' parsing from a 'pair' to a 'struct' to make further improvements easier

Convert attribute 'target' parsing from a 'pair' to a 'struct' to make further 
improvements easier

The attribute 'target' parse function previously returned a pair. Convert 
this to a 'pair' in order to add more functionality, and improve usability.

Differential Revision: https://reviews.llvm.org/D35574

llvm-svn: 308357